A lightly regarded American team turned in a gutty performance on the final lap of yesterday's 45-mile team trials to win a bronze medal in the last event of the Junior World cycling championships.

The medal was the first ever won by the United States in any international amateur bicycle race, including the Olympics.

As expected. East Germany and the Soviet Union repeated last year's finish by placing first and second, respectively, in the race along the Maryland portion of the George Washington Parkway. East Germany collected a total of eight medals for the seven-event championships; the Soviets followed with six.

To win the bronze, the quartet of Greg LeMond, Greg Demgen, Jeff Bradley and Ron Kiefel had to make up a seven-second deficit to pass Italy on the final lap. They toured that 11 1/4 miles in 23 minutes 54 seconds, a final-lap clocking exceeded only by East Germany and Norway.

Then the Americans had to sweat out the performance of the Italians, last year's bronze winners who had started the race 25 minutes after them. The Americans became optimistic when the Italians appeared over the hill 400 yards from the finish line being passed by the Soviets, who themselves had started three minutes behind the Italians. When Italy's time was announced the stunned crowd of about 1,000 erupted into delighted cheering for the Americans.

Italy had faltered so badly on the final lap (25 minutes flat) that it lost fourth place to Norway.

"They (the Americans) were shot at the finish, dead tired, just like they should have after riding a good race," said Bill Humphries, assistant U.S. coach. "After the third lap of four, we were just trying to place in the top six. The third place is just fantastic."

Asked about the team's thoughts during the final lap, Demgen replied, "We were just trying as hard as we could, with everyone working hard. Near the end, we wanted the lead man (in the single-file formation) to be the one who felt the strongest at the time." Demgen and LeMond took turns leading over the final quarter-lap.

The Americans had gotten off to a shaky beginning when, 20 seconds before their scheduled start, Kiefel's chain jumped off. That problem was remedied with about seven seconds to spare. Otherwise the clock would have been started at the designated starting time even though the U.S. than that, the team suffered no mechanical problems in the race.

Other follies at the start included a disagreement between the driver of Italy's support truck and an official. Told he had too many personnel in the truck, the driver got out, tossed off a new expletives in his native language, and then had to run to catch his vehicle, which had drifted perilously close to the Italian team waiting at the starting line.

Not to be topped was the Soviet support force, whose truck died and refused to start at the starting line.After trying to jump-star the truck, the Soviet finally had the announcer appeal to the crowd for the use of a van. They finally caught up with their cyclists on lap two.

Top 10 Finishers

East Germany, 1:31:03; Soviet Union 1:33:08; United States, 1:35:06; Norway, 1:35:39; Italy, 1:36:06; West Germany, 1:36:14; Switzerland, 1:36:40; Denmark, 1:36:14; Sweden, 1:37:18; Austria, 1:37:49.