What was going on when I was born?

Nestled in a lushly upholstered chair in his Park Avenue office, Toshihiro Tomabechi, the 65-year-old president of the Mitsubishi International Corporation, marveled the other day at how much things have changed since he first came to the United States in 1951. Then, Mr. Tomabechi was one of a few dozen Japanese businessmen and diplomats in New York. He represented one of the Mitsubishi group's ''splinter firms,'' created in 1947 when the Allied command broke up the powerful Japanese families of companies known as zaibatsu, which literally means ''wealthy clique.'' In 1951, with World War II still a vivid memory, anti-Japanese sentiment was widespread and, by law, Japanese stationed in America were prohibited from bringing their families here. ''It was not until May 1953 that my family could join me,'' Mr. Tomabechi recalled.

The Canadian Air Traffic Controllers Association announced last night that its members would no longer handle air traffic to and from the United States, except in emergencies. The union said it had acted because of what it regards as unsafe conditions created by the six-day-old strike of American controllers. At a meeting in Ottawa, the union announced that beginning at 7 A.M. today scores of flights to or from the United States "would not be processed." The union cited what it said were 41 documented incidents of aircraft flying too close together and nine instances of "evasive action" taken by aircraft in Canadian airspace. It also referred to "amateurish attempts" in the United States to run an air traffic control service" with "unqualified substitute personnel."

Water in America: Solving the Quandary Second of five articles While New Yorkers suffered through a drought emergency this spring and reservoir levels sank dangerously low, basements in parts of Queens were flooded with some of the 1,500 billion gallons of water that lies, mostly untapped, beneath that part of the city. At the same time, as millions of New Jersey and Connecticut residents underwent mandatory water restrictions, billions of gallons of water were overflowing from reservoirs in the area and running off into the ocean. In July vandalism cut off Newark from its major reservoirs and left the city of 600,000 people with less than two weeks of water. Engineers worked day and night to restore the rupture and complete pipes to nearby water systems that, almost all agree, should have already been in place. According to most experts, the towns and cities in New York metropolitan area have, at present, no water supply problems so great that they cannot be solved with careful planning, basic conservation measures and better systems of maintenance, control and coordination.

Three months after the start of a 50-cent surcharge on nighttime and Sunday taxi fares, the Taxi and Limousine Commission has been unable to get any data from the independent cab drivers on whether more drivers have been encouraged to work during those times. Without this data from the independent drivers, who lobbied hard for the surcharge, the 50-cent nighttime and Sunday surcharge may be scuttled, the chairman of the commission warned yesterday. The official, Jay L. Turoff, said the fleet owners - representing only 20 percent of the city's 11,787 medallions -had provided statistics showing that drivers had been making an average of 2.3 extra trips each evening and that the number of cabs remaining in garages for lack of drivers had been cut in half. But he said he could not draw definite conclusions from those numbers. And he said the commission did not have the manpower to do a complete survey of its own on the need for the extra taxis.

International One of the toughest Soviet attacks on the Reagan Administration followed the report over the weekend that the United States would proceed with the full production of neutron weapons. Tass, the official Soviet news agency, said the American move was ''a step designed to bring the world closer to nuclear catastrophe,'' and that the Soviet Union would respond in the manner required by its own security interests. (Page A1, Column 6.) West Germany responded cautiously to the Reagan Administration's plans to produce neutron weapons. A Government spokesman said that the decision was an ''exclusively American affair.'' Influential members of Chancellor Helmut Schmidt's Social Democratic Party who believe that the Administration is not taking disarmament efforts seriously indicated there would be increased resistance to support of its armaments policy. (A6:3-6.)

Developing countries have made considerable progress helping themselves, yet they face an uncertain future because of the slower growth of world trade and less generous aid policies of industrial countries, the World Bank said today. In its fourth annual world development report, the bank drew attention to the ''increasingly desperate predicament'' of the poorest countries and made a plea for open trade policies as the catalyst for growth for both the industrialized world and the developing world. The 192-page analysis was prepared by the staff of the International Bank for Reconstruction and Development (the World Bank's official name) under the direction of Robert S. McNamara, who retired as its president June 30. Data from the report will be studied by industrial and developing countries before their conference at Cancun, Mexico, in October.

From the highways, the events seemed like another weekend carnival or a revival meeting. Balloons swayed in the air to the rhythm of country music. Crowds milled around under colorfully striped tents. And announcers with the cadence of auctioneers welcomed the multitudes to the midway. But instead of carnival wares or personal salvation the organizers were selling 3,000 new cars built by the Chrysler Corporation. With the bait of $300 to $1,000 a car in rebates during the month of August, the sale was billed as the largest such promotion in automotive history.

Major league baseball clattered back onto the national scene after eight weeks tonight with a display of razzle dazzle and a burst of home runs that raised the curtain on the second half of the split season. But the long, strikebound summer did nothing to reverse the course of a series dominated for a generation by the National League. In a game watched by a record crowd of 72,086 fans and played by a record total of 56 men, the Nationals hit four home runs and rallied to defeat the American League, 5-4. It was their 10th straight victory and their 18th in the last 19 games. And they did it this time with four home runs: two by Gary Carter of the Montreal Expos, who was voted the game's outstanding player, and one each by Dave Parker of the Pittsburgh Pirates and Mike Schmidt of the Philadelphia Phillies.

Gypsy-moth caterpillars did a record amount of damage in the Northeast in 1981, eating the leaves off trees covering at least nine million acres from Maine to Maryland, according to estimates by forestry experts. That was twice the area defoliated last year, they said. "This is the greatest defoliation or infestation that has ever been recorded in the Northeastern United States," said Peter W. Orr, an official of the United States Forest Service at its regional office in Broomall, Pa. In New York State, 2.4 million acres were damaged by the moths and the Hudson Valley was among the areas hardest hit.

One day, about a hundred years from now, some mice may be born of fathers and mothers that lived and died here in 1981. These 21st-century mice with 20th-century parents will be withdrawals, so to speak, from one fo the world's strangest bank accounts: the embryo bank of the Jackson Laboratory. Until they are needed for research, the mouse embryos will be kept frozen at minus 196 degrees centigrade in liquid nitrogen refrigerator tanks. Their intended destiny is to be thawed, then gestated in foster mothers and born as living mice for use in scientific research. While mice ordinarily live two or three years at most, embryos thawed after eight frozen years have grown into normal mice.

President Reagan is being urged to decontrol prices on all domestically produced natural gas by the beginning of 1985, rather than just a portion of it. The move is expected to meet heavy Congressional opposition because of higher costs to the consumer, but it has a fair chance of gaining the President's support because of his recent budget and tax victories. (Page A1.)

Oil ministers from the 13-nation Organization of Petroleum Exporting Countries will convene a special meeting Aug. 19 in Geneva, a senior Kuwaiti official said Sunday. The meeting will try to unify OPEC oil prices and end the current supply surplus on world markets, according to Abdel Rahman al-Awadi, Acting State Minister for Cabinet Affairs. Saudi Arabia, which produces 40 percent of OPEC's total oil output and whose high production levels are the main cause of the current oversupply, has agreed to attend the meeting, he said.