Arena Stage has been awarded $310,000, the first installment in a five-year grant from the National Endowment for the Arts that will allow the 34-year-old theater to provide year-round employment for the members of its acting company and expand its artistic resources.

The grant, announced yesterday, is the largest of eight that the NEA has made to theaters across the country as part of its newly inaugurated Ongoing Ensembles program.

Arena will be required to come up with matching funds over the five-year period, at the end of which the combination of the sliding NEA grant and income raised by the theater itself will amount to $2,065,000.

"I believe this is the most important grant the NEA has ever given," Zelda Fichandler, Arena's producing director, said yesterday. "We in the regional theater all started out with the notion of forming resident companies. For a number of reasons, we've been through a long period where the company idea has become an anomaly. But theater is a team game and you do it better when you have a team. This is going to help make that the rule."

Arena now has a resident company of eight actors, who are employed for 35 to 40 weeks a year. (Other actors are hired on a show-by-show basis.) With the NEA grant, which becomes available on June 1, 1985, the theater will be able to increase the resident company to 15 to 18 actors, all on year-round contracts, Fichandler said. (Arena's budget for the upcoming season is $6.2 million, 73 percent of which is provided by ticket sales.)

Arena does not intend to add to the number of plays it presents each season, which runs from mid-September through May, but the grant will permit the company to remain together during the summer months, instead of turning to outside employment. That extra time, Fichandler said, can now be devoted to a variety of training programs, workshops and in-depth rehearsal periods. She also plans to use some of the grant money to bring playwrights to Arena on an extended basis to develop new works. Lighting, scenic and costume designers -- many of whom have been hired in the past for a one-shot assignment -- can now become an integral part of the Arena company, she said.

Fifty-one theaters applied for the initial NEA grants, totaling $1.3 million this year. After Arena, the Circle Repertory Company in New York received the second-highest grant, $260,000. Other theaters receiving the special NEA funds are: the Milwaukee Repertory Theater ($65,000); The Roadside Theater, Whitesburg, Ky. ($60,000); the San Francisco Mime Troupe ($90,000); the Spanish Theater Repertory Company, New York ($120,000); Trinity Square Repertory Company, Providence, R.I. ($220,000); and The Wooster Group, New York ($170,000).

Each theater is obliged to re-apply at the end of the year for the next installment of the grant. According to a graduated formula, the NEA provides the bulk of the funds initially, but with each succeeding year the size of the grant diminishes and the respective theater is obliged to raise an increasingly larger sum. In Arena's case, it must match the $310,000 the first year with $103,000. By the fifth year, the NEA grant will be $103,000 and Arena's contribution $310,000.

In awarding Arena Stage the grant, NEA officials took into account the theater's endowment campaign, which was launched last May and to date has raised approximately $3 million (of a targeted $6 million). The Arena endowment will begin bearing interest in 1988 and that money, Fichandler said, would be used in part to meet the financial requirements of the NEA grant. "But the next two years are going to be tight," she said.

At the ceremony held at Arena yesterday to announce the NEA grant, Hugh Southern, NEA deputy chairman for programs, praised Arena as "the flagship theater of the not-for-profit professional theater movement . . . the envy of and a beacon for the rest of the theater field." He added that the grant would allow its "already outstanding work to achieve new heights of quality."

The regional theater movement has greatly altered the face of American theater in the past 30 years, largely replacing Broadway as the nurturer and presenter of serious dramatic works. But while it was originally hailed as an alternative to the commercial theater -- freeing actors, among others, from the pressures of the New York marketplace -- that promise has only been partially fulfilled. Each year, for example, Arena loses several of its company members who decide to pursue a riskier, but potentially more lucrative, career in either New York or Los Angeles. Some geographically isolated regional theaters have severe difficulty recruiting actors who prefer to remain close to the major centers of film, television and theater.

"The past has proven the positive value of a group of artists -- actors, directors, playwrights, designers, and others -- working together as an ensemble over a period of years, with each project building on the last," NEA Chairman Francis Hodsoll has said. "Due to a lack of emphatic financial support, this approach has suffered steady erosion over the first 30 years of the resident theater in America . . . The disappearance of the ensemble as a viable alternative would be extremely detrimental to the art of the theater." The NEA Ongoing Ensembles program focuses on the problem by allowing resident theaters to raise the salaries of their personnel and broaden the artistic challenges confronting them.

A spokeswoman for the NEA said that the eight theater grants were not financed at the expense of other NEA programs. Because of an increase in congressional appropriations, the NEA's budget for theater rose from $9.4 million to $10.6 million this year. The extra funds were used for the Ongoing Ensembles program.

Last week, Arena was also the recipient of a $265,000 NEA support grant, one of 191 awards made by the Endowment to nonprofit professional theaters throughout the country.