Almost 10 years to the day, Leonard Bernstein's "Mass" is amassed once again at the Kennedy Center Opera House, where it opens tonight.

The enormous array of forces required by the work that launched the center on Sept. 8, 1971, has been matched once again. Logistically, there's nothing else quite like "Mass." What other "theater piece" these days takes 250 players -- that's one for approximately every 10 in the audience? Or requires an adult choir of 58 to 78? Or a boys' choir of 28? Or a 20-member street chorus? Or a rock band and a blues band? Or two separate stars to handle the grueling role of the celebrant? It could almost be a staged version of one of the large Mahler symphonies that Bernstein conducts so well.

"Nothing else but opera costs like this," says Kennedy Center chairman Roger L. Stevens of the new production. "It's pretty darned hard to do this for a profit. When we redid the first one, we finally got back our production costs and made a little, but just a little."

The estimated gross cost of this "Mass" is $975,000. "That's from the first postage stamp onwards," says Charlene Harrington, the production coordinator.

It is midafternoon and the cast, chorus and orchestra are rehearsing in their street clothes -- a distinction without much of a difference in this work, because many of the players are costumed for the production in what look like street clothes anyway. It is the day of the first preview and much is going on because Bernstein has arrived in town and has seen the show for the first time the night before. "We were up until 4:30 in the morning going over it," says one associate. "Lenny went through the whole show with director Tom O'Horgan of "Hair" and "Jesus Christ Superstar" and discussed what was going on." Now it was time to work on changes.

O'Horgan and conductor John Mauceri had not gone through their plans with Bernstein since taking the set model up to his home in Connecticut for approval. Mauceri, though, had worked with Bernstein on "Mass" for nine years and conducted the European premiere in Vienna.

During the opening Kyrie eleison, producer Harrington points to the stage. "This part is very hard," she says. "We've been rehearsing those street groups for two weeks, in rehearsal halls and right up to here. The problem is the size of the groups."

"One of the toughest things about staging 'Mass,' " explains O'Horgan, "is simply traffic. We build up these huge groups on the stage and then the problem is to get them off." He has had some success in trimming back the appearance of throngs that are standing but not performing. "We've tried to simplify the organization of them by focusing with lights and different groupings."

Even the considerable technical capacity of the opera house equipment is pushed. "Mass" has 300 lighting cues, which are programmed by computer and monitored on video display terminals offstage and in the lighting room at the rear of the orchestra. All one need do is press a button there for a new set of changes in the precisely structured sequence. In addition, there are five follow spotlights. At the rehearsal, lights are still being adjusted.

Harrington recalls Monday when they started moving in after the much simpler sets and equipment for "Annie" had been struck. "The stage hands finished 'Annie,' then got some sleep and came back the next morning to start this. It took 16 hours just to hang all of these lights. Actually, the Kennedy Center's crew is one of the best in the country or it would have been worse."

Fortunately, "Mass" involves no set changes. It is done in one act and lasts a little more than an hour and a half. And O'Horgan's new production looks less visually complicated than the earlier one under Gordon Davidson.

O'Horgan began work on the production when it appeared it would be performed in concert halls instead of an opera house. "We had planned to try it out in Carnegie Hall, bring it on to the Kennedy Center Concert Hall and then do some touring," he explains. Thus it had to be more flexible than the earlier staging.

But Stevens said that touring was ruled out because of the additional expenses of traveling with so large a cast. One source of money, though, will be the live telecast Sept. 19 on PBS.

Meanwhile, Stevens is keeping a loose rein on the show. As of yesterday afternoon, he said, "I haven't even seen it. I've been busy, and I didn't feel Mr. Bernstein needed my advice."