The owner of the Houston Rockets relied on his 21-year-old daughter to call "heads" yesterday for the most important coin flip in the NBA since 1969, a decision that will enable the Rockets to select Virginia's Ralph Sampson with the first pick of the June 28 draft.

"I was shaking so much, I thought I was dancing," said Charlie Thomas, the Rockets' owner.

The Indiana Pacers, who lost the coin flip, had turned down several offers from Houston to trade the Pacers' first pick for draft choices and players. But Indiana officials said before the coin was tossed that they'd take the 50-50 chance.

It turned out to be a major miscalculation and allowed Houston to walk away with the right to draft Sampson, a 7-foot-4 center who three times was college basketball's player of the year.

NBA sources say it may cost the Rockets $1 million a year in salary for as many as five years to sign Sampson. A new collective bargaining agreement that provides for a team salary cap goes into effect in the NBA in 1984, but it will have no bearing on Sampson's contract negotiations.

NBA Commissioner Larry O'Brien started the session in his midtown office by tossing a 100-year old silver dollar to decide who would call the real flip. After winning the preliminary (Houston was assigned heads), Thomas, on the advice of his daughter Tracy, said, "I guess I'll call heads again."

Indiana owner Herb Simon looked on helplessly as the silver piece came up heads again.

Jim Foley, the Rockets' public relations man, ripped off his coat and shirt, flashing a T-shirt with "RALPH" across the chest. "I would have given up my religion if we had lost," Foley said.

Sampson, meanwhile, was saying nothing publicly. All interview requests were taken by Doug Elgin, director of sports information at Virginia. Elgin released a short statement that said Sampson did not want to discuss the coin flip until after graduation ceremonies in Charlottesville on Sunday.

Houston, which had the worst record in the NBA (14-68), will draft Sampson with the first pick in the NBA draft. Indiana, worst in the Eastern Conference (20-62), may select Steve Stipanovich, a 6-11 center from Missouri, who outplayed Sampson in a game last season. But he is not expected to have Sampson's impact in the next decade. Indiana officials said yesterday they might consider trading the pick.

"If we had gotten to make the call, I feel we would have won," Simon said. "I wish we could have gone down fighting."

Most NBA scouts seem to agree that Sampson is the best center to come out of college since Bill Walton of UCLA in 1974, or Lew Alcindor (now Kareem Abdul-Jabbar) of UCLA in 1969.

The third selection in the draft also belongs to Houston. But since this is the weakest draft pool since 1977, the Rockets may trade the pick for future draft choices or a veteran player or players. If the Rockets keep the third pick, they will probably select a power forward.

After each of the past three seasons, NBA teams reportedly were ready to offer Sampson millions to turn professional, but he declined. He nearly left Virginia after his junior year, but was deterred largely because of the coin toss (between Los Angeles and San Diego).

Within hours of the flip, Rockets fans had flooded the team's offices with orders for about 300 season tickets. "Everyone is so excited," said Cathy Bartley, the assistant general manager. "When they call, they ask who won (the flip). Then they start screaming and drop the phone. Then they'll come back and ask for tickets."