The two brightest track and field stars of the 1976 olympics, Lasse Viren and Alberto Juantorena, are fading, Juantorena to the point where he might not step onto the track here.

A day after insisting to a wire service reporter that he would defend both his 400- and 800-meter titles, Juantorena scratched from the 800 heats today. There are indications an Achilles' tendon injury that required surgery in March also will keep him from the Sunday morning 400 competition.

One source said the surgical scar on Juantorena's right heel is especially ugly and that "el Caballo" is here primarily as a goodwill representative of Cuba. If Juantorena decides to compete in the 400 it would be his first race since the operation.

Juantorena became the first runner to successfully double in the 400 and 800 in the Olympics, running remarkably well for a man with the build of a National Football League tight end. His feat was supposed to mean the world would see other oversized halfmilers, but that has not happened.

Viren always has a mysterious air about him -- and the 10,000-meter trial tonight was no different. He was tightly bunched with four others on the lead with slightly more than three laps to go, then suddenly slowed.

It appeared as though Viren might be making a token trip to the finish and saving his energy for the marathon. Then an odd thing happened: he could not avoid qualifying for the 10,000 final.

That came about when Ireland's John Treacy, one of the four pulling steadily away from Viren, suffered dehydration and heat exhaustion and collapsed about 250 yards from the finish line. Viren, perhaps 70 yards behind Treacy when he fell, ended as the fourth automatic qualifier.

"I went very well for the first 7,000 meters, but after that I was disappointed," said Viren, who celebrated his 31st birthday in the Olympic Village four days ago. "It was hot and muggy out there."

Viren, who won the 5,000 and 10,000 titles in 1972 and '76 and finished fifth in the marathon at Montreal, could scratch from the 10,000 final Sunday evening if he is as out of shape as he appeared, and concentrate on the Aug. 1 marathon.

Or he might recover well enough to run exceptionally well in both. With Viren, little is certain.