What was going on when I was born?

''I think we have to do it to demonstrate something to the whole world.'' - Oscar Gonzalez, voting in El Salvador. (A1:6.)

A headline on Sunday incorrectly stated the rate at which medical costs in the United States are climbing. The nation's health-care bill has climbed tenfold in the last two decades, from $27 billion in 1960 to $275 billion last year.

Coleco Industries, a leading maker of games and toys, was the best performing stock on the New York Stock Exchange in the first quarter, rising 96.4 percent. The Texas International Company, an oil and gas exploration concern, turned in the poorest showing, dropping 65.7 percent. A list of the best and worst performers on the Big Board, the American Stock Exchange and the over-the-counter markets is on page D4.

In the Great Tin Mystery - a $500 million business who-done-it - the clues all lead to Kuala Lumpur, the capital of the world's largest tin producer. Malaysia has neither confirmed that it was the kingpin in the plot to corner tin, nor denied that its warehouses are full of the key metal. But those who follow the trading of tin have no doubt that Malaysia was involved. Seen from Malaysia's point of view, the bizarre actions of the past eight months do make sense. Malaysia did, and still does, have a lot to gain from the uncertainty that grips the world tin market.

The Legislature is about to pass what some view as a spend-now, pay-later budget. News analysis, page B3. By JOYCE PURNICK Mayor Koch yesterday criticized as ''counterproductive'' an agreement by the leadership of the State Legislature that would delay some financing for the Westway project. ''This project cannot be stopped,'' he declared. ''It will ultimately be built.''

While Americans overwhelmingly are pleased with the quality of their medical care, most appear to be willing to accept significant changes in the system to reduce costs, according to a national survey. Probably the most striking conclusion of the poll by The New York Times was the extent to which people expressed concern about rising medical costs and seemed open to trying the various means that have been proposed to control those costs. For example, nearly 6 of every 10 said they would be willing to have routine illnesses treated by nurses or doctor's assistants rather than doctors. And, by a ratio of 50 to 42 percent, they indicated they would agree to use a clinic, where they would have to accept any available doctor, rather than go to a private doctor, if that would help economize. This is the approach used by prepaid group plans, or health maintenance organizations, which have been widely touted as a cure for inflation of medical costs.

Middle-sized companies in most older industrial areas seem to be riding out the recession, while some larger ones are having difficulty paying their bank loans. Banks on the West Coast have run into loan problems because of the depressed housing market, which has hit real-estate developers and related industries. (Page D1.)

Voters, some casting their ballots to the sounds of nearby gunshots, jammed polling places across El Salvador today in elections for a constituent assembly. Guerrilla forces managed to keep almost all voting locations closed in Usulutan, the country's third largest city, and voting was reported light in San Francisco Gotera, where two United States observers, Representative John P. Murtha, Democrat of Pennsylvania, and the Rev. Theodore M. Hesburgh, president of Notre Dame University, were unable to land their helicopter because of fighting. Elsewhere in the largely rebel-dominated Morazan Province, however, turnout exceeded expectations. Early morning attacks in other provinces delayed the opening of polling places, but once government control was restored, voters thronged to them.

Last Wednesday, after her first match in the $300,000 Avon women's tennis championships, Sylvia Hanika was asked if anyone could beat Martina Navratilova in the season-ending event. After all, Miss Navratilova had not lost a match all year. ''I think if I were playing her, and I played like this, she would have to play very well to beat me,'' was her reply. ''She doesn't like to play topspin.''

FRANK McGUIRE, Ben Carnevale and Bob Spear, then the head basketball coaches at North Carolina, Navy and Air Force, were sitting in a hotel room in March 1958 when McGuire mentioned he was looking for an assistant coach to replace his old friend, Buck Freeman, who was ill. McGuire, who is here for the National Collegiate Athletic Association tournament, reconstructed that gathering of coaches yesterday. McGuire recalled that Carnevale, who coached North Carolina to the N.C.A.A. title game in 1946, said, ''Bob here has a young assistant who is about to get out of the Air Force and will be looking for a job.'' Spear praised the young man, Dean Smith, and McGuire said: ''I come from New York where there are a lot of Smiths. But they're Bill Smith, Sam Smith, Frank Smith. What is this name - Dean? Can't we change it?''

As demonstrations continued in the West Bank today, the Government affirmed its policies in the occupied territories, declaring that ''no acts of violence or disturbances of the peace or order will be tolerated.'' In northern Israel, leaders of Israeli Arabs prepared for a general strike and demonstration on Tuesday to demand an end to the Israeli occupation in the West Bank and Gaza and the formation of a Palestinian state. (Page A5.) Three Palestinian protesters were shot and wounded by Israeli soldiers in Yabed, near the West Bank town of Jenin. An army spokesman said they had attacked a patrol with knives and Molotov cocktails. According to an army statement two of the Palestinians were hospitalized. No soldiers were injured.

After a six-month investigation involving 77 witnesses, a grand jury in Newark is expected to vote Wednesday on whether to indict Mayor Kenneth A. Gibson for purportedly giving a no-show job to a former City Councilman, according to the Essex County Prosecutor. ''We've presented all the evidence to the grand jury,'' said the Prosecutor, George L. Schneider. ''Now the ball's in their court. They'll decide on Wednesday.''