Rodney Jenkins and Idle Dice, who have given the world of show jumping many memorable moments, returned tonight to the Baltimore Internationale Jumping Classic after a 2 1/2-year separation and won the $3,000 open jumper stake.

Jenkins, 37, of Orange, Va., was one of 16 entries in an original field of 25 to survive the first round of competition over a grueling course of 13 fences to qualify for the timed jumpoff. Jenkins also qualified on his other two mounts, Semi Tough and Second Balcony, both out of the money in the final standing.

However, when he entered the arena on Harry Gill's 18-year-old bay gelding and made the turn toward the first of seven fences in an abbreviated course, it was as if they had never been apart. Jenkins pushed down his hard hat, as was his habit for many years, and went full tilt. He finished with no faults in 29.37 seconds.

During the 1970s Jenkins and Idle Dice dominated the Grand Prix jumping circuit from Florida to New York, gathering an unmatched $300,000 in earnings along the way. Then 2 1/2 years ago he and owner Gill had a misunderstanding and parted company.

Earlier this year before the Florida circuit Gill approached Jenkins to ride Idle Dice again. "He got him fit on the farm and the year he had off was good," said Jenkins. "In his first 12 years of showing he never had three months off. He's dead sound. We don't even use Butazolidin."

Donald Cheska of Hartland, Wis., finished second on South Side in 29.64 seconds. Bernie Traurig, also of Hartland, on Edenvale was third in 30.20 seconds.

Jenkins, currently ranked 10th on the list of North American World Cup qualifiers, said he and other riders from the United States plan to boycott the 1981 World Cup finals later this month in Birmingham, England, because, "The host country does not plan on paying our expenses to compete. Last year when the United States served as host it paid for everything."