The decision by the National Collegiate Athletic Association to go ahead with tonight's NCAA basketball championship game between Indiana and North Carolina was made less than 45 minutes before it was to begin in the Spectrum.

Wayne Duke, chairman of the NCAA basketball tournament committee, said the 5:08 p.m. third-place game was allowed to start because it was thought President Reagan's condition was not critical following an attempted assassination today in Washington.

As doubts arose about the president's condition and the Academy Awards were postponed, NCAA tournament and Spectrum officials met at halftime of that game and decided to hold a final meeting following its completion.

That meeting included NCAA officers, the presidents of two universities playing for the championship, other school officials, all members of the NCAA executive committee present, the nine-man NCAA tournament committee and Spectrum officials. Duke said the decision to play the game was unanimous.

Duke said reports that the president was out of surgery and recovering played a big part in the decision to go ahead. NBC Television, which televises the NCAA tournament for a $9.95 million annual rights fee, had said earlier it would broadcast the game unless the president's condition deteriorated seriously. Duke said NBC had no input in the decision to play.

Don Ohlmeyer, executive producer of NBC Sports, said the network's only conversation with the NCAA was to relay its position. Arthur Watson, president of NBC Sports, said cancellation of the game or a network decision not to televise it would have cost NBC "several million dollars" in advertising spots, sold by the network at $200,000 per minute.

However, Watson said, the network would not cut away from its coverage of the attempted assassination unless Reagan was out of surgery and recovering. "You just can't cut away from it to go to a damned basketball game," he said in the NBC control trailer outside the Spectrum.

Sources close to Watson said the decision to telecast the basketball game was made at the highest network level, by Fred Silverman, president of NBC Television.

It was still unclear tonight whether the NCAA would have received the full amount of its payment from NBC had the game not been televised. Officials from both the NCAA and NBC said the contract contains an "impossibility of performance" clause. A decision by NBC News to maintain continuous coverage of the president's condition would have fallen under that clause and made the contract voidable.

But Don Wear, counsel for NBC, said, "We wouldn't have acted unilaterally or dictatorially and invoked the clause. We would have discussed it with the NCAA before arriving at a decision as to whether or how much we would pay."

NBC recently lost its NCAA tournament contract to CBS. NBC will continue to televise regular-season games and has exclusive contracts through TVS and other independent producers for games played by almost all of the nation's top conferences. The network is making strenuous efforts to recapture the NCAA tournament contract.

First sure word that the game would be played came as UNC Coach Dean Smith stood outside his locker room talking to the press about 7:45. Indiana Coach Bob Knight interrupted him. They huddled in a tunnel leading to the floor. As they walked back, Knight was heard asking Smith, "What time are you going out?"

At that point, according to Smith, Knight was told by IU Athletic Director Ralph Floyd that the game would be played. Smith then went into the locker room for a pregame meeting with his team. Both he and Knight said their players had not been informed that there was a possibility the game would be postponed.

UNC players were on the court at 7:30 shooting informally.

At his press conference, Duke said that at the first meeting tonight, the possibility of declaring the teams co-champions was discussed. But, at the second meeting, the only talk was of whether to postpone or go ahead. No possible new date was discussed in that meeting.

Duke said that both coaches were contacted separately and told the NCAA that they would go along with whatever the tournament committee decided. Members of the UNC coaching staff reportedly thought that the game should go ahead.

During the telecast, announcer Billy Packer indicated that he had spoken with assistant coaches for both schools, who told him the players knew about the assassination attempt, but were not "distracted in their game preparation."

The days events forced Ohlmeyer to rearrange the format for the telecast.

NBC cut away from its coverage of Reagan's shooting at 8:14. The telecast from the Spectrum had been scheduled to start at 8. Announcer Bryant Gumbel came on and said the game had been "tempered considerably by the events of this afternoon in Washington, D.C."

He went on to say that the network had decided to televise the game because of the news that Reagan was not in danger and because the NCAA had opted to play the game.

"Our hearts and players are with the president," he said. There was a brief prayer before the game, and 10 seconds of silence.

All three announcers for the game, Packer, play-by-play man Dick Enberg and Al McGuire, said they would have preferred not to go on. Near the end of the game, Enberg said, "There are much more meaningful things happening in this country of ours."

The opening tipoff and the early moments of the game were not shown because of an NBC News update.

"This is not the easiest assignment in the world to handle," Gumbel said before the game. "Whatever you do, some people will say you did too much and some people will say you didn't do enough. I just want to do what is right. . . You know, there are some people that couldn't care that Reagan was shot. They just want to see the basketball game."

Ironically, during a news update late in the game, NBC News reported that the president was awake and had joked, "All in all, I'd rather be in Philadelphia."