Four weightlifters--Cubans Daniel Nunez and Alberto Blanco Fernandez and Canadians Guy Greavette and Michael Viau--were stripped today of their Pan American Games medals and the records they set because tests showed they had used drugs.

Apparently, many more Pan Am athletes face similar discipline.

Urinalyses of three other weightlifters who did not win medals showed they, too, had used anabolic steroids and their cases were referred to the International Weightlifting Federation for action. They were Francisco Guillermo Lopez of Argentina, Jose Lozada of Puerto Rico and Caballero M. Dolcey of Colombia.

All seven could be declared ineligible for the Olympics.

Further disqualifications are expected in the next few days, with reports prevalent that at least one United States weightlifter will be included. The U.S. Olympic Committee called an 11 a.m. news conference for Tuesday, with indications it would deal with the drug situation.

Tonight, for the second time in two weeks, a press conference advertised as of "the highest importance" was canceled by the USOC.

Men's track coach John Randolph, scheduled to meet the media at 10:30 p.m., returned to his hotel at 11 and said, "Nothing will be said tonight, because there's nothing to be said yet. I have to go through the chain of command but I'm still planning to say what I have to say tomorrow morning."

Speculation was that Randolph intended to complain about the low caliber of his team and was overruled by the USOC.

Cuba's Nunez won--and lost--three gold medals in the 60-kilo (132-pound) weight class here and in the process set a world record of 130 kilos in the snatch. He was a triple gold medalist as a bantamweight in the 1979 Pan Am Games. Fernandez at 220 pounds won three gold medals, light heavyweight Greavette two golds and a silver, and lightweight Viau three bronze medals.

Officials said it was the second time in the 32 years of the Pan Am Games that athletes had lost medals because of drug use. In 1979, Canadian Joan Wenzel had to return her bronze medal in the 800 meters when it was found she had taken an antihistamine, to treat a cold.

The weightlifting competition ended Friday and the medal winners, plus two other athletes from each event, were tested by a West German laboratory that supposedly has the ability to detect any use of steroids within the past six months.

Many weightlifters have depended on steroids in the past, only to stop their use a month or six weeks before a competition with testing, thereby avoiding detection.

Today's announcement, made by Eduardo Henrique de Rose, president of the Pan American Games medical commission, meant Cuba had its gold-medal total in weightlifting reduced from 23 to 17 and Canada lost two of its three golds.

In competition tonight, Ricardo Prado of Brazil won his second gold medal in swimming, but the U.S. squad responded with four consecutive triumphs and a third world record to end the 29-race meet with 25 gold medals. Counting diving, the United States' aquatic total was 29 golds in 33 events.

The 1979 U.S. Pan Am team won 28 of 29 gold medals.

The U.S. boxing team completed an 11-0 sweep of the preliminary round, with victories by 132-pounder Pernell Whitaker and Jerry Page, 139, assuring that everyone on the team will go home with a medal, since all semifinalists earn at least a bronze.

Whitaker, world amateur champion from Norfolk, Va., knocked out Kenworth Minus of the Bahamas at 1:50 of the first round. Page, of Columbus, Ohio, won a 4-1 decision over South American champion Jose Magallanes of Venezuela.

The unbeaten U.S. baseball team beat Venezuela, 10-3, as USC junior Mark Gwire hit his sixth home run in five games. Cuba continued toward a showdown with the U.S. by defeating the Dominican Republic, 15-5, for its 29th straight Pan Am victory.

In the final event of the swim meet, the U.S. 4x100 medley relay team set a world record of 3 minutes 40.42 seconds.

Rick Carey, Steve Lundquist, Matt Gribble and Rowdy Gaines improved on a mark of 3:40.84 set by the same team in Ecuador in the World Championships a year ago. Carey, who set the tone for the race by swimming his leadoff leg in 55.60, and Lundquist set world records earlier in the meet.

Prado, who earlier won the gold in the 400-meter individual medley, led throughout the 200-individual medley race and finished in 2:04.51.

Mary T. Meagher rallied to beat teammate Tracy Caulkins for the women's 200 butterfly gold in 2:10.06. Amy White, 14, set a Pan Am record during a driving rain in the women's 200-meter backstroke. Teammate Sue Walsh pushed the young Californian to her first major triumph, leading until the last 50 meters, when White came on to win in 2:15.66.

Mario Vasquez Rana, president of the PASO, said that, for the last three days, officials had automatically taken the gold, silver and bronze medalists, plus two other athletes from each event, for urinalysis and that such testing would continue through the end of the competition.

Previously, he said, athletes had been picked at random for urinalysis.

"This is some of the saddest information I've ever given," Vasquez told a news conference. "Something to regret has happened, and we have to take care of it."

He indicated that perhaps as many as 50 athletes may be involved.