Rick Carey broke the world record in the 100-meter backstroke for the second time in the day when he won the event in 55.38 seconds last night at the U.S. Long Course Swimming Championships in Clovis, Calif.

Later Steve Lundquist, an SMU graduate fron Jonesboro, Ga., set a world record of 1:02.34 in the 100 breaststroke.

Carey and butterfly specialist Matt Gribble had set world records in the morning preliminaries.

Carey set his second world record in four days when he swam a 55.44 in a qualifying heat in the 100 backstroke. About an hour later, Gribble lowered the 100 butterfly record to 53.44.

Carey set his third record soon after Tiffany Cohen of Mission Viejo, Calif., won the women's 1500 freestyle in 16:11.97, the best time in the world this year. The Cohen, 16, became the meet's first triple winner.

Carey took the 100 backstroke record away from John Naber, America's swimming hero at the 1976 Olympics. On Wednesday, he broke Naber's 200 back record.

"I was just 13 years old when John set those records," Carey said. "They've been around as long I've been swimming competitively" . . .

In Bartlesville, Okla., Greg Louganis won his 24th U.S. national title, coming from behind on his sixth dive to win the platform final of the U.S. Outdoor Championships.

Louganis, 23, who lives in Mission Viejo, Calif., got 86.13 points on the sixth dive, a reverse 1 1/2 somersault with 3 1/2 twists, and increased his margin with 83.52 points on his seventh, an inward 3 1/2 somersault.

Louganis had 643.59 points to defending champion Bruce Kimball's 625.29 and Ron Meyer's 623.79.

Michele Mitchell, also of Mission Viejo, won her first U.S. title on the platform, recording scores of 7 or above from all the judges on all her dives.