What was going on when I was born?

The Kaiser Steel Corporation said today that it had accepted a ''definitive'' acquisition bid valued at $270.8 million from a group of investors led by Irwin L. Jacobs, the Minneapolis industrialist. The new offer is practically identical to a bid that had been accepted by Kaiser on May 31 and which Mr. Jacobs pulled away from on June 30. The new offer, however, includes a $23 million payment from Kaiser to the Jacobs group for its 16.3 percent stake in the company. The payment would be made after the takeover is approved by the other shareholders.

The Administration today announced an agreement with Moscow committing the Soviet Union to buy at least nine million metric tons of American grain in each of the next five years. This is 50 percent more than it was obligated to purchase in the past. ''This is a happy hour for American agriculture and for the citizens of this country,'' said John R. Block, the Secretary of Agriculture, in announcing the accord. He said it was ''symbolic of the normalization of trade in agriculture commodities'' with the Soviet Union that the Administration had sought.

A fire here this week shot poisonous chemicals into the air, keeping 750 people from their homes for as long as four days, sending more than 125 of them to the hospital for checkups and leaving the community wondering what, if any, long-term health effects residents may face. Helicopters have buzzed the area all week, rushing scientists and lab samples back and forth between Albany and the town of Rose, with a population of 2,672, just south of Lake Ontario, halfway between Syracuse and Rochester. Two dozen state health, environmental and agricultural specialists have swarmed over Rose and neighboring Clyde and Wolcott, studying the possible effects the released toxic chemicals may have had on the people, the ground water and the crops downwind from the chemical fire. Nothing is certain. While state officials say they are confident that any immediate health risks have passed, they are less sure about long-term implications. And as the evacuation order was lifted today and residents began to return, the consensus was that little Rose had never seen so much excitement. People did not know whether to worry or to make light of it all.

A substance routinely added to the steam lines at Cornell University has extensively coated artwork in the Herbert F. Johnson Museum here, and there have been numerous health complaints from museum workers. More than half of the 40 members on the staff of the museum have reported illnesses they say they believe are associated with exposure to the chemical. They have called on the university to close the museum during a cleanup, which is scheduled to begin next week. Much of the $10 million collection, with sculpture, ceramics and American, Asian and European paintings, was coated with an oily film. Museum officials said the extent of damage to the art had not been determined.

The George Brett home run that was, then wasn't, is. The case of Brett's pine-tarred bat slipped in another direction yesterday when Lee MacPhail, the American League president, upheld a protest by Brett's team, the Kansas City Royals. He overruled the umpires at the game, who had canceled the two-run home run Brett hit in the ninth inning at Yankee Stadium last Sunday on the ground that he had too much pine tar on his bat. MacPhail's ruling marked the first time he had upheld a protest in his 10 years in office, a period in which perhaps 50 cases of protest had been sent to him. It also wiped out the Yankees' 4-3 victory and created a game that is suspended with the Kansas City Royals leading, 5-4, with two out in their half of the ninth. In addition, it knocked the Yankees out of a first-place tie with Baltimore in the highly Text of the ruling, page A16 contested division race in the American League East. MacPhail acknowledged that Brett had pine tar too high on his bat, which made the bat illegal. But he based his decision reinstating the home run on other factors, including what he and rules-makers saw as the intent of the pine-tar rule and his belief that ''games should be won and lost on the playing field - not through technicalities of the rules.''

When night falls, owls and moths, snails and mice come out to play or prey at the New York Botanical Gardens in the Bronx, but most humans don't get to see them because the place is closed at night. Tonight at 7, however, Urban Park Rangers will lead a night tour in the gardens, pointing out not only sights, but also sounds and smells. For instance, some crickets chirp faster as the weather gets hotter, so if you know the formula, you can determine the temperature. Also for instance, the sassafras tree's leaves smell like root beer and the black birch's like wintergreen. The tour will also explore a glacier's path and other bits of natural history. Participation is limited and registration required: 548-7880. BUSY DAY FOR CLARINET As part of the Mostly Mozart Festival, the clarinetist Richard Stoltzman offers a free half-hour concert today at 1 in Lincoln Center's North Plaza. He'll play works by Stravinsky unaccompanied, then jazz pieces accompanied by his brother Kent Stoltzman, an amateur bassist. This will be their first public performance together. Tonight at 8, the clarinetist will be the soloist in the Stamitz Clarinet Concerto No. 10 in B flat, and Yefim Bronfman will be the soloist in Mozart's Piano Concerto No. 15 in B flat (K. 450). Haydn and Mozart symphonies are also on tap. Tickets: $8.50 and $11. Information: 874-2424.

The Israeli Army touched off widespread protests today by ordering the Christian Phalangist militia to close at least one of its offices and barracks in southern Lebanon. Phalangist officials said the Israelis had also ordered two other militia bases closed and had vowed to remove the militia from other areas as well. Thousands of Christians, in a cluster of villages around Sidon and Nabatiye, rang church bells, closed their shops and blocked roads with smoldering tires, burning cars and barbed wire. In the Beirut area, 200 Phalangist supporters marched on the Israeli military spokesman's office at Yarze and demanded that the order be rescinded.

Marc Rich & Company A.G., a major international commodities trader, paid $1,350,000 yesterday toward a $50,000-a-day fine that began June 29. The continuing daily fine, paid in Federal District Court in Manhattan, was imposed by Judge Leonard B. Sand for the company's refusal to provide records that were subpoenaed for a Federal grand jury's investigation of the company for possible tax evasion. On July 15, Judge Sand ordered the company to pay the $1 million in fines that had accrued by July 18 and then to pay $200,000 each Friday and $150,000 each Monday for as long as the company failed to turn over the records.

An article in Metropolitan Report on Monday about Federal outlays for New York State incorrectly attributed a suggestion for a study of the Fed- eral-state fiscal relationship. The sug- gestion, for the Rockefeller Institute of Government at the State University of New York to assist in the study, was made by Governor Cuomo.

BETTY CARTER, the most resourceful, musical and eloquent singer in jazz today, is opening this year's Summerpier series of jazz concerts tonight at 8 o'clock at the just-reopened South Street Seaport. Admission is free. You can simply pack a blanket or a chair, take the IRT express, Nos. 2, 4 or 5, to Fulton Street (or call 669-9400 for alternative directions) and sit listening to Miss Carter's virtuosic scat-singing while the Seaport's historic sailing vessels rock gently at anchor and traffic rumbles a low-key counterpoint from the spidery heights of the Brooklyn Bridge. If that sounds like a cultural mixed metaphor - well, so is New York City. And in the summer, the mixed metaphor moves outdoors. With the start of the Summerpier series, New York's outdoor jazz and pop concert season hits full stride. Of course, it isn't necessary to go to a concert to enjoy music outdoors. There are street musicians to be heard, some of them very fine, in many midtown parks and in open plazas on sunny weekdays. One jazz group seen performing recently, near the Avenue of the Americas and the 42d Street stop of the IND subway, included musicians with substantial reputations, at least one of whom has recorded several albums.

Something new has been added to this genteel landscape of fields, woods, New England clapboards and mountains that define the horizon like walls of green pillows. What is new is a solid piece of New York, N.Y.: Carlos from East 148th Street, Alfie from Fort Washington Avenue, Teresa from West 204th Street, Angela from Sunnyside and 39 other New York City public-school children, all of them between the ages of 9 and 14, living and working at Bennington College this month in pursuit of muses both classical and unorthodox. The youngsters are here under the auspices of a Manhattan-based nonprofit group called the Arts Connection, studying dance and what their teachers call ''circus arts.'' Since the beginning of the month, it has been a summer of practice, practice, practice, hour after hour of tumbling, juggling and plieing, day after day of hard-earned fun. It is the kind of fun, say the children, that they would not likely find at home.

Prices plunged and yields rose for long-term Government securities and corporate bonds yesterday after Paul A. Volcker, chairman of the Federal Reserve, said that corporate credit needs were beginning to clash with Treasury borrowings to finance the Federal deficit. Short-term interest rates posted more modest rises of about onetenth of a percentage point. It was the intermediate and longer maturities that bore the brunt of persistent late selling. Yields of these longer-term securities rose one-quarter of a point or more.