Drugs again held the spotlight at the Pan American Games today as a Dominican Republic sprinter was censured for having taken a banned substance and the executive director of the U.S. Olympic Committee confirmed that several United States athletes had tested positive before coming here.

The revelations came on a day when Cliff Wiley reached far back for the inspiration he needed to win the 400 meters in 45.02. His victory averted a shutout for the United States in track and field competition.

Juan Nunez, a silver medalist in the 100 meters, became the 15th athlete to be named in the drug crackdown when a postevent test conducted by the Medical Commission of the Pan American Sports Organization found a considerable amount of the drug phencamphamine in his system. He was reported to the Dominican Olympic Committee and the International Amateur Athletics Federation for possible sanctions.

Phencamphamine is a stimulant similar to an amphetamine that affects the central nervous system. It is one of about 100 drugs on the banned list.

There was no decision whether Nunez must return his medal. So far, 22 medals have been returned by athletes found guilty of using banned drugs.

F. Don Miller, executive director of the USOC, said several U.S. athletes competed in the Pan American Games even after most of them had failed precompetition drug tests.

Miller said, "It was their individual decision whether to participate or not participate. As far as the USOC is concerned, I advised the athletes if I were taking banned substances, in my judgment--knowing the sophisticated equipment--I would choose not to participate."

(William Simon, USOC president, told The Washington Post today in a phone interview from his home in East Hampton, N.Y., that reports of athletes' pretesting at the Pan American games were "essentially correct.

("Don Miller met with the athletes to brief them and they asked if they could be tested in a 'personal', kind of doctor-patient way," he said. "The doctor said they could be screened--that's what it was, a screening, not a complete testing. Then, if they tested positive, it would be up to the individual to go on."

(Simon said his understanding of the situation regarding weight lifter Jeff Michels of Chicago, whose three gold medals were taken away after he was said to have tested positively for steroids, was that "the first test done on Jeff was negative, and that was for steroids. Then the second time, he was positive, and I believe that test was for testosterone."

(The drug testing being done at Caracas, according to Simon, "is apparently able to pick up all the banned drugs, but I'm not qualified to comment further on it."

(Simon returned to the United States earlier in the week with a dozen track and field athletes who withdrew from the Games. He said the athletes he spoke with in Caracas "have no sympathy for guys who are taking something and get caught with it. It destroys the basis of equity in competition, and now I think the game is over. We've gotten our message across."

(The USOC, he said, will take over the drug program and establish procedures "whatever the best, whatever the cost" to stamp out drug use.)

After Wiley finished first, he jogged a victory lap and then had to lie down on the track to regain his breath.

"When you run the quarter, you exert your body to its farthest point," he said. "I think I reached that point. I made it through the win and the victory lap but then it kind of caught up with me."

Wiley, 28, said he has had an asthma condition for some time and added, "I've had some problems breathing down here, with the altitude and high humidity."

Until Wiley surged past Cuban Lazaro Martinez in the stretch, it appeared that Cuba was going to continue the trend that has brought it to the forefront in the second week of Pan Am competition.

The Cubans won two of the six gold medals at stake today, for a total of 10 with one day of track competition left. The U.S. has eight.

Another potential Cuban success was negated in the homestretch when Charmaine Crooks, a Canadian who attends Texas-El Paso, overtook Ana Quirot to win the women's 400 in 51.49. Easter Gabriel, the only U.S. entry, was third.

The men's triple jump and women's discus resulted in 1-2 finishes for Cuba. Jorge Reyna won the triple at 55 feet 11 inches, with David Siler of Tennessee, the only U.S. competitor, fourth at 52-10. Maria Betancourt was the discus champion at 197-8; Lorna Griffin of the United States salvaged the bronze at 185-5.

Until today, the highlight of Wiley's track season was his graduation from University of Kansas law school. The U.S. champion the previous two years, he was sixth this year and did not qualify for the World Championships. So he went back whence he came, the streets of Baltimore.

"This as been a tough year for me," he said. "At the beginning of the season, I thought I had a chace for a great year. But it had trouble with my strength, then there was everything connected with graduation and finally I pulled in practice a month before the TAC meet.

"I competed in Europe, not very well, and then I came home and trained for this one. I went back to my roots in Baltimore and trained on my old tracks. I talked to my old high school coach and my old recreation coach. To them I'm the only big-time athlete in the world and they gave me my confidence back."

The race did not begin auspiciously for Wiley: he was charged with the second of two false starts. On the third try, he sat in the blocks until the gun, started a slower than usual, then turned it on in the stretch, making his move as teammate James Rolle was tying up and fading to fourth.

In basketball, the gold-medal winning U.S. men's and women's teams finished undefeated with easy victories over Puerto Rico. Wayman Tisdale of Oklahoma had 20 points in the men's 101-85 victory and Lynette Woodard, formerly of Kansas, scored 20 as the women won, 112-65. The men finished 8-0, the women 5-0.

Brian Masters, 19, of Columbia, Md., and Insook Bhushan of Aurora, Calif., won the singles titles in table tennis. And Andrea Godin, an athletic trainer at George Mason University, won a silver medal in the 48-kilo class of sambo wrestling.