Mary Decker Slaney made her 1985 outdoor track and field debut tonight by breaking her own American women's record in the 5,000 meters before a cheering hometown crowd at the Prefontaine Classic.

Slaney, running competitively outdoors for the first time since colliding with Zola Budd at the Los Angeles Olympics last summer, won in 15 minutes 6.53 seconds. Although Slaney broke her old American mark of 15:08.26 set in the Prefontaine meet three years ago, she fell nearly eight seconds short of her premeet goal, Ingrid Kristiansen of Norway's world mark of 14:58.89.

Slaney's time was the fastest in the event this year by 17 seconds.

Earlier, Jarmila Kratochvilova ran the fastest women's 800 meters in the world this year.

The world record holder in the 400 and 800 meters, Kratochvilova was expected to get tough competition from Robin Campbell and Joetta Clark of the United States. But she won easily in 1:58.01. Campbell finished second (2:00.62) and Clark's time was 2:02.59.

Before the race Kratochvilova was concerned about her injuries. "I had an injury in January," she said, "but mostly I have a problem with my maximum speed because I am not as fast as I was before I was 30."

Sydney Maree outsprinted Doug Padilla down the stretch to take the men's 5,000 meters in 13:20.48, by far the fastest time in the event this year and his personal best.

Joaquim Cruz, the Olympic gold medalist at 800 meters, held off a challenge by Steve Scott and Jim Spivey to win the 1,500 meters in 3:35.70 seconds.

This was the fourth time Cruz and Scott have competed against each other. Cruz has won three times, Scott once. "I don't see Scott as my only rival," Cruz said.

The 200 meters went to Calvin Smith of Bolton, Miss., who finished in 20.68 seconds. The race was run under a steady drizzle, but that didn't bother Smith.

The 3,000-meter steeplechase was won by American record holder Henry Marsh of Salt Lake City in 8:20.50. Marsh pulled ahead of Olympic bronze medalist Brian Diemer 80 meters from the finish.

This was the 10th anniversary Prefontaine Classic, which is named after Oregon distance runner Steve Prefontaine, an Olympian who died in an auto accident 10 years ago.

This year's Pre classic is included in the Mobil Grand Prix circuit, an international meeting of world-class track and field athletes, began last week with the Bruce Jenner Classic in San Jose. The Grand Prix consists of 15 meets in 13 countries and carries $542,000 in awards.