O'Neill Center nurtures new voices in American theater.

'Tis the season for summer theater. And for many patrons, alfresco dramatics means sitting through clouds of mosquitoes and endless productions of ''Same Time Next Year.''

But at the Eugene O'Neill Theater Center, a nine-acre former seaside estate here in the hometown of its namesake, a unique tradition has been forged - not with summer stock revivals but out of the new voices in American theater.

''We're involved in changing attitudes, an attempt to educate people away from the idea that the show that should be seen is the one you can't get a ticket for,'' says Lloyd Richards, dean of the Yale School of Drama and artistic director of the O'Neill Center. ''Taking that chance (with) a playwright, to keep up the flow of new work, that's really the mission for us.''

Every summer for the past 20 years, a dozen or so talented but little-known playwrights have arrived at the O'Neill to hone their craft, away from the marketplace, among the sea gulls and copper beech trees. Using a unique four-day workshop process culminating in staged readings open to the public, the O'Neill's annual National Playwrights Conference has served as an idyllic cultural nursery for such playwrights as Lanford Wilson, John Guare, and Sam Shepard. Such works as Guare's ''House of Blue Leaves,'' John Pielmeiers's ''Agnes of God,'' and John Patrick Shanley's ''Danny and the Deep Blue Sea'' got their start on the verdant O'Neill grounds.

This summer, the center celebrated its two-decades-old tradition by playing host to an International Playwrights Conference. As more than 500 previous O'Neill participants and patrons cracked lobsters and hobnobbed under a gaily striped tent, actors, playwrights, and directors from Denmark, Venezuela, Australia, the Soviet Union, and China gathered to take part in the O'Neill process.

In an atmosphere reminiscent of an artists' colony, where the rural stillness is broken only by the clack of typewriters and the murmur of actors rehearsing their lines, the O'Neill puts its formula to work: four days of rehearsals, two book-in-hand staged readings with limited sets, and a morning-after critique. It's a process that permits the playwright to rewrite right up until the minute of performance. It has also become a model for other cultural institutions, including the Australian National Playwrights Conference and Robert Redford's Sundance Institute in Utah.

While some participants occasionally chafe at the workshop restrictions - ''Sometimes there just isn't enough time between the rehearsals and performances for a writer to make all the corrections he needs,'' says one director - most playwrights are nothing but enthusiastic.

''The first time I heard my play read through, it felt like I was watching a car skid off the road,'' confessed one writer. ''But the chance to be the audience to your own work - that's the real value here.''

''You write in isolation and always the big question is whether what is in your head is dramatically right,'' says George Hutchison, an Australian playwright and one of the participants in the O'Neill's 20th anniversary International Playwrights Conference. ''The only way is to see it in performance. This workshopping is an invaluable process.''

Although any number of environments now operate as cultural launching pads for novice playwrights, from New York's New Dramatists to Kentucky's Actors Theater of Louisville, few such opportunities existed back in 1964 when George White, the O'Neill founder and current president, first got the idea for a playwrights' conference and cashed in his life insurance policy to fund it.

''It was originally going to be a summer adjunct of Yale's drama school,'' says Mr. White, enjoying one of the perks of his job as he held a meeting on the lawn. ''This is a New England town, and a lot of people thought it was going to be summer stock. But by 1967 we were into totally unknown playwrights, and a lot of people were shocked (by the plays' explicitness), including myself. A couple of times the (local residents) tried to close us down.''

By 1970, however, with Lloyd Richards at the artistic helm, the O'Neill was well on its way to establishing its nearly peerless reputation as a developer of new playwrights. ''Early on, we were known as 'Try-out-town USA,' '' White says. ''On weekends, limos from New York would arrive and disgorge people from (producer) David Merrick's office.''

Along the way, the center also became something of a summer hot spot for up-and-coming actors and directors. Al Pacino, Meryl Streep, Michael Douglas, Swoosie Kurtz, and Howard Rollins have all spent time on the O'Neill boards. Among other additions are a New Drama for Television Project and a month-long Critics Institute. The O'Neill emphasis is never one of performance for performance' sake, however. ''Audience reaction is simply one of the steps in the (playwriting) process,'' says Richards, who five years into the program forbade outside critical reviews. Today, only regional theater directors are invited to come and scout out the 12 or 15 new works.

That dozen or so plays represents about 1 percent of the 1,500 annually submitted. ''There aren't that many good plays around,'' one observer commented. ''If they read 1,500 here, that's 1,500 that the regional theaters don't have to read.''

''Lot's of theaters are developmental, but they do it for themselves and not for the field,'' Mr. Richards says. ''The O'Neill should be a home for (new playwrights). That's what separates us.'' The Center, whose $400,000 annual budget is met by private and public funding, is also one of the few theaters that takes no part in a playwright's future earnings, a practice common to many regional and Off Broadway theaters.

''It's more of a home here,'' says playwright John Patrick Shanley, a two-time O'Neill participant whose ''Danny and the Deep Blue Sea'' - now playing Off Broadway - was first done at the O'Neill last summer. ''They're committed to the writer here.''

''Without the O'Neill process I would still be writing plays and I would still be writing the same plays. But they would not be as developed,'' says writer August Wilson, whose ''Ma Rainey's Black Bottom'' will open on Broadway this October and was first read at the O'Neill two years ago. ''I am hoping to write another one between now and December so I can come back next year.''