Leonard Bernstein is nestled into his seat in the darkened Jones Hall for the Performing Arts, overseeing a final dress rehearsal yesterday of his new opera, "A Quiet Place," which will have its premiere here Friday night.

It is a somber work about a torn family and the pain of death and is Bernstein's boldest and most demanding undertaking since "Mass," which he composed for the inauguration of the Kennedy Center in 1971. And it is a work that reflects a somber Leonard Bernstein--a man who continues to mourn the death of his wife five years ago, a man who is feeling the toll of advancing years and therefore feels the need to channel more of his energies into composing.

As he sits in Jones Hall, Bernstein, who will soon be 65, is surrounded by associates in this project, many of them less than half his age.

Supposedly he is just an observer of the large cast of singers and the enormous orchestra as they proceed with the performance. But Bernstein is as animated as anyone in the hall. When the music gathers rhythmic impetus his conducting arm goes into action, with the lighted cigarette in his fingers tracing the beat in the darkness. He shoots directions to the staff like a benign buzz saw; when a singer's work especially impresses him he shouts "Brava" or "Bravo" at the top of his lungs. There are roaring guffaws at punch lines. During a break, he leaps down the aisle to embrace the director. It has been like this for most of the day.

Work on "A Quiet Place" has been proceeding briskly since Bernstein arrived over the Memorial Day weekend in Houston, where the resourceful Houston Grand Opera has taken on the challenge, and the enormous expense, of launching the new opera, which it commissioned in a joint arrangement with the Kennedy Center and La Scala in Milan. It will come to the Kennedy Center in October.

The composer has kept almost entirely to his hotel suite and the rehearsal room of Jones Hall, intensely absorbed in what he calls "our great workshop in the sky." The only place where he regularly mingles with the public is at the hotel pool, where the Texas sun has stained him a dark reddish brown.

Bernstein has declared that "A Quiet Place," a one-act opera in four scenes lasting about an hour and 45 minutes ("longer than 'Salome,' " he blurted with glee), is "unlike any work I have ever written or seen." After an initial viewing the observer is left with little ground for contradiction.

The idea is a simple one--to take the troubled suburban family that is the subject of "Trouble in Tahiti," his chamber opera of 1952, and pick up on it again in 1983. "Trouble in Tahiti," which has gained a secure place in American musical theater, will be performed here as a curtain-raiser to "A Quiet Place." The title of the new work derives from the plaintive aria in "Trouble in Tahiti" sung by the wife and mother, Dinah, from a psychiatrist's couch hoping for salvation for her marriage: "Then love will lead us to a quiet place."

"Trouble in Tahiti" ends with the future of the marriage of Dinah and Sam very much in doubt. Thus the viewer is set up for the shock delivered as the curtain goes up on "A Quiet Place."

The scene is a funeral parlor. The marriage has gone on in its empty way for 31 years, and Dinah's earlier anxiety has slowly turned to despair and, now, suicide. She has just drunk herself into a stupor and driven her car over a cliff, leaving behind a note "To Whom It May Concern."

Sam stands silently with his back to the audience for most of the lengthy funeral parlor scene, as friends of the family sing in a complex interwoven sequence of dialogues about the tragedy and their memories of Dinah. The two children, Junior and Dede, long since estranged from their parents, have not yet arrived. They are being joined by Franc,ois, who is Dede's husband and Junior's lover. Junior, now 40, has become a pathetic pistol-toting psychotic who makes a scene in the funeral parlor, finally knocking down the lid of the open coffin.

The subject is the wreckage of the family in contemporary America and the theme is the necessity to reestablish communications. Yet if the tone of the libretto and the music is often abrasive, the direction of the work, nonetheless, is ultimately hopeful.

Librettist Stephen Wadsworth's text is full of intricate word plays--word games are a symptom of Junior's illness. Franc,ois retreats into his native French when under stress. The opera is full of four-letter words, and the knotty sequence of dialogues in the funeral parlor scene gives Bernstein little chance for a lyric flight in the voices. That comes in the second scene, back at the family home that evening. In a lengthy double duet that grows into a large-scale quartet, Sam and Dede go through Dinah's things in one bedroom while Franc,ois tries to calm Junior in another.

As Houston Grand Opera director David Gockley observed, "Sometimes this opera is less like 'West Side Story' than it is like Virgil Thomson and Gertrude Stein." Thomson and Stein collaborated on "Four Saints in Three Acts" and "The Mother of Us All."

Underlying it all is an orchestral fabric in a wide variety of styles that is of truly symphonic density--the opposite of "Trouble in Tahiti." Bernstein compared the four-part shape of the opera to a Mahler symphony in an interview with a Houston critic last week. "The opening scene is huge and explosive. The second is elegaic. The third is a playful scherzo," he said. And the last scene is "one of those adagios," referring to the grave and noble slow movements that conclude works like the Mahler Third and Ninth symphonies. "If the opera is saying anything," he said, "it is saying that anything in life is hard to achieve." Then he added, "including this opera."

Bernstein seems increasingly reflective as age etches its lines on the face of music's most celebrated Wunderkind. He is granting no interviews to the two dozen music critics who have gathered here for the premiere, but he talked about age and his eventual musical legacy in a taped discussion here with a Chicago radio interviewer several weeks ago.

"I don't create as fast as before," he said. "I broke off conducting last October to work on this opera and I didn't write a note for a month. I had to change my point of view totally. It was different when I was composing back in the '40s, for instance, when I could sit down and write in train stations or wherever I happened to be.

"It is more important now, in however many years I may have left, that I compose rather than I conduct. There are younger men who conduct some things better than I anyway . . . I don't know what will be thought of my music in the future, but composing is different from conducting because those are my notes and nobody else's."

The collaboration of Bernstein and Wadsworth on "A Quiet Place" grew from common needs to write musical works dealing with the subject of losing a beloved family member. In Bernstein's case it arose from the death, from cancer, of his wife Felicia Montealegre, and in Wadsworth's the source was the death of his sister in a car crash.

The two had met previously, but the collaboration came about by chance in 1980. Wadsworth was managing editor of the magazine Opera News, and had been asked by the Saturday Review to interview Bernstein.

"'Let's do the interview,' " Wadsworth recalls he wrote to Bernstein. " 'I'm not a bad guy, and we'll have a good time and if you don't want to do it, or if it's going to bore you, I'll leave or we'll play anagrams'--which I knew he liked, and so do I--'or we'll go to the movies and forget about it.' "

"And then, in a moment which I will never forget, I wrote at the bottom, 'P.S. Interested in librettos?' "

To his astonishment, Bernstein called, saying he wanted to pursue a sequel to "Trouble in Tahiti." He told Wadsworth to listen to a recording of it, " 'and if you give me a scenario for a sequel by Tuesday at 4 o'clock'--it was Friday--'I will give you an interview. Fair trade?' " Wadsworth delivered on time.

It was only later that they discovered the coincidence of the deaths in their families. As events developed, Wadsworth continues, "we found ourselves both wanting (a) to write an opera and (b) to write an American opera--that is to say, in the verbal/musical/emotional American vernacular, and (c) to express something about families coping with death."

Asked how much "A Quiet Place" is autobiographical for either of them, Wadsworth says, "The piece is autobiographical, I like to say, in spirit. It is not literally autobiographical."

Houston has provided Bernstein with an unusually attractive nesting place for the birth of "A Quiet Place." There is less of the pressure that complicated things for him with "Mass" at the Kennedy Center. Yet he has at his disposal the sophisticated staff and technical facilities of an opera company that in only 25 years has developed into an institution of international status.

Under the direction of Gockley, who is 39, the Houston Grand Opera in the last decade has had wide impact. Houston's productions of "Porgy and Bess," "Treemonisha" and "Show Boat" have all come to the Kennedy Center, as well as Carlisle Floyd's "Willie Stark," which was commissioned by the Houston opera.

Houston has spared little introducing "A Quiet Place." Rigorous workshops were held in New York and at the University of Indiana where Bernstein and Wadsworth worked things out with performers. Bringing the production to the stage costs about $500,000, says Gockley, and it will take an additional $300,000 "to run."

Starting Friday night "A Quiet Place" will receive nine performances here in 12 days. During that time, says Gockley, "I think Bernstein feels that the opera will be in a continually evolving state. That will give him a couple of weeks when he can work with it. It will be interesting to see what happens to it after the end of its run here and before it opens at the Kennedy Center."

Gockley is asked if "A Quiet Place" will stand up musically and theatrically. He answers guardedly, "I don't think we know yet."