LAST MONTH, the National Opera Institute was in the Kennedy Center's Theater Lab to stage two fledgling American operas. Tonight, the institute will be in the Terrace Theater, awarding $35,000 to young American singers.

In between, the group has been doing its usual job: finding and furthering Yankee ingenuity in musical theater, particularly opera. It's a daunting task, considering the Europeans' age-old dominance. But since its founding in 1969--by Roger Stevens and the legendary baritone George London--the group has devoted itself to promoting the careers of American singers, composers, librettists, producers, directors, conductors and even makeup artists and wig-makers.

The Metropolitan Opera's current installment of "Tales of Hoffmann," one of the most acclaimed productions in that company's recent history, boasts five singers--including Catherine Malfitano, Gwendolyn Bradley and Claudia Catania--who got grants from the institute early in their careers. So did the stage director, Lesley Koenig. The list of other noted beneficiaries, from Met soprano Ruth Welting to Washington Opera chorus master William Huckaby, is as long as your average Wagnerian epic.

Many confirmed opera lovers have never heard of the National Opera Institute--and with good reason. Its six full-time staffers work in a stuffy, windowless dressing area, partitioned into cubby holes, in the Kennedy Center's basement. Far from public view, the group is suitably small and low-key. But, by most accounts, it has clout and influence beyond its size.

"For opera in this country, it's the Brookings Institution," said James Ireland, assistant director of the Houston Grand Opera. "If there's a new problem or a new idea, they can wrestle with it by putting together a committee of experts. They can encourage opera companies to produce new works. They can invest in the future by helping individual singers. It's really American opera's think-tank."

The institute's board of trustees is a Who's Who of musical theater: opera stars Leontyne Price and Sherrill Milnes, Broadway lyricist Sheldon Harnick, New York City Opera director Beverly Sills and the eminent producer Harold Prince, who does both operas and musicals. He's serving this year as chairman.

"I'm terribly concerned about the future of American musicals and operas, which are very much interlaced," said Prince, whose recent productions include the likes of "Sweeney Todd" and "Evita." "How could I be sanguine? Not enough new operas being written by Americans are entering the repertoire. They may get one performance, and that's it. There are a lot of opera companies across the country, but they may not know how to locate and stimulate the writing of new musical theater pieces. That's one of the things we're here for."

The institute's mixed bag of money, of which $725,000 will be spent this year, comes partly from private donations, partly from the National Endowment for the Arts. The Kennedy Center's Roger Stevens was chairman of the endowment when he established the institute as an independent entity--"It seemed like a good project at the time," he said laconically--and hired George London to be its first executive director.

London, a tall, dark baritone with a volcanic personality, held the post until 1977. That year, John Ludwig, an experienced opera administrator, took over, and London, who had departed to head the Washington Opera, suffered a near-fatal heart attack that left him severely disabled.

Under London's stewardship, the institute built a reputation for nurturing singers--not least because he'd often give them coaching free of charge. He urged singers to spend their grants, in amounts from $2,000 to $5,000, as practically as possible. That might have meant clothes and lessons, as with soprano Georgine Resick, or a medically supervised diet, as with tenor John Aler. Both now have flourishing careers.

"George had a great gift for working with singers," said Ruth Sickafus, who has managed the insitute's office since it opened a dozen years ago. "If he heard somebody that he thought had extraordinary potential, there was a compulsion to help them. He had an uncanny ability to listen to a singer and put his finger on the potential problem, then point the way to correcting the problem."

Since Ludwig came to the institute from the San Francisco Opera, where he was artistic administrator, he has broadened the group's image and added new programs: think tanks on matters such as television and education, supporting new works and trying them out with professionals. He has also broadened the group's focus to include popular musical comedy.

"Our purpose," said Ludwig, "has always been to forward the progress of all kinds of music theater." He is a medium-sized, middle-aged man with a slightly worried look. "We deal with new pieces and entry-level professionals who are not necessarily young, but youngish. We look for the most talented composers, librettists, stage managers, designers, singers and whatnot, and we put some money into their lives."

The institute's 5-year-old "music theater workshop" program, Ludwig's pride and joy, lets composers and librettists see their creations without the expense of a full production--perhaps $4 million in the case of a Broadway show. The program's track record is impressive. Henry Mollicone's "Starbird," William Mayer's "A Death in the Family" and Carlisle Floyd's "Willie Stark"--chosen for workshops in the last few years--all caught the fancy of opera producers.

"Willie Stark," which made the biggest splash after its workshop in 1980, premiered at the Houston Grand Opera (Harold Prince produced), played the Kennedy Center's Opera House and was televised by PBS. Now Floyd is publishing the score, in hopes that other companies will do it.

"I can't tell you how valuable the 'Willie Stark' workshop was," said Floyd, whose works include such contemporary classics as "Of Mice and Men" and "Susannah." He'll be at the Terrace Theater tonight to pick up the institute's "Service to American Opera" award.

"Not too many impresarios have any interest in new opera," he said. "Whenever a new work does not succeed, it gives ammunition to people who would rather not do them anyway. The workshops are relatively cheap--a good way of trying to safeguard that ultimately very large investment."

With costumes and set, a workshop can run about $60,000. In the past, the institute has sent the money to an established opera company like Houston's or Minnesota's for the actual production. This year, for the first time, the institute is playing producer, with Ludwig taking an active part in the theatrical nuts and bolts.

"We wanted to raise our profile a bit," he said. "Now that we're getting less from the National Endowment"--whose contribution has shrunk to about 40 percent of the budget--"it's a good strategy for attracting some corporate donors."

Last month--first at New York's TOMI Theater, then in the Kennedy Center's Theater Lab--the institute gave workshops, at a cost of about $120,000, to "The Virgin Unmasked" and "Amarantha." The money came from the National Endowment and Exxon.

"The Virgin" is a musical comedy by two Minneapolis-based authors: Sharon Holland, who wrote book and lyrics, and composer Hiram Titus. "Amarantha," from a short story by Wilbur Daniel Steele, is an opera by Roger Ames, a former Washingtonian who lives with his wife--the aforementioned Georgine Resick--in Dusseldorf, Germany.

The Theater Lab devoted a marathon weekend to the works--"The Virgin" got three performances and "Amarantha" two. Both changed considerably from first performance to last, as the authors pared some scenes and added others in a frenzy of revision and creation. "Amarantha," which Ames finished writing only weeks before rehearsals, metamorphosed more--and caused the bigger stir.

Scored at present for two pianos, percussion and a dozen "singing actors," it sports tensely melodic music--"I hear traditionally when I compose," Ames said--and a story line about a country girl's brush with a crazy but high-minded killer.

"The best thing about the process is that it's like getting ready for a Broadway opening without having to open," said Ames, a towering bear of a man with a curly mane and salt-and-pepper beard. "You go through all those agonizing, very desperate moments and weeks to get the piece working, then you move away from it to take a long hard look."

Revising substantially from performance to performance, Ames spent two weeks canvassing audiences in New York and Washington, consulting with the actor-singers, and holding high councils with the production chiefs. Forsaking sleep, he spent hours by himself at the keyboard with a pencil.

"We locked him in a room and didn't open the door, except to bring him a sandwich," joked the opera's music director, Paulette Haupt-Nolen, a member of the institute's workshop panel, who brought Ames' piece to Ludwig's attention. Also director of the Lake George, N.Y., Opera Festival, she will give it a world premiere in 1984.

By the last performance, "Amarantha" had a new opening scene, a closing aria for the title character and expanded roles for a few others--until the opera had grown from one act to two.

"It's very powerful," Sharon Holland told him at the closing night party at the Kennedy Center. Standing together in a clutch of admirers, the authors of the two operas looked a mixture of exhaustion and elation. "Well, we did it, didn't we?" Holland said.

"How do you feel?" Ames asked, grinning and wiping his brow.

"Ready to go home and give it a rest."

"I'm going to get right to it," the composer said briskly. "While it's still fresh."