WHEN THE American Theater Critics Association voted to hold its annual convention this year in Nashville, there was some astonishment in the ranks.

Theater in Nashville? Heart of country music? What theater would there be to criticize?

Retired from his long tenure as drama critic of The New York Times, Brooks Atkinson maintained that judgmental independence which distinguished his career: "Getting away from the obvious theater centers is the brightest idea this outfit has had."

Atkinson was referring to the kind of provincial thinking that assumes theater exists in only a couple of metropolitan areas and is ignorant of just how thoroughly theater has penetrated throughout the land since the creation in 1964 of the arts endowment. The Tennessee meeting would prove him right.

For several generations, Nashville's principal theatrical focus was the Ryman Auditorium, loosely adapted from its years as a church. Major stars -- Hayes, Cornell, Lunt and Fontanne -- all played there and told anecdotes about its colorful manager, Mrs. L. C. Naff, who stuffed everyone's tickets into her vast reticule and jammed 14 ticket-buyers into pews intended for 10.

There was a professional community theater that had such leading ladies as Shirley Booth, Blanche Noyes (the Washington aviator), Dinah Shore and Hank Fort. But the Grand Ole Opry gradually took over the Ryman, and the city became the hub of country music recording. In such a setting, how much theater could there be?

Charged with setting the record straight was ATCA's executive committee chairman, Clara Hieronymus, arts editor of the Nashville Tennessean. She summoned up theater groups from Memphis to Johnson City, Knoxville and Kentucky, as well as a bus expedition to Cumberland Playhouse, 120 miles from Nashville.

Most important was the new Tennessee Performing Arts Center, the first such complex created by a state. Opened last fall, the center is housed in the 18-story James K. Polk State Office Building near the Capitol. In the uniquely American liaison between government and the private sector, the state financed the $42 million structure, while programming is being underwritten by the Tennessee Performing Arts Foundation.

Besides the State Museum, the building includes three theaters named for Tennessee's presidential trio. Andrew Jackson Hall, seating 2,442, is home for the Nashville Symphony, major touring productions and occasional operas. The James K. Polk Theater with similar stage facilites and seating 1,056, is used for smaller productions and concerts. The Andrew-Johnson Theater, accommodating 300, is in the "black box" style, permitting any seating arrangement a director chooses. As with all new operations, the challenge is to convince managers that its stages are viable and audiences willing, an undertaking that always takes time.

In the scene of events scheduled over a four-day period in May, the distinct find, insofar as many cities were concerned, was three rare actors of the Roadside Company of Whitesburg, Ky. They played a tale from old court records, "Red Fox, Second Hangin.' "The long-forgotten but intricately researched trial would be a winner anywhere, so deft are the three actors in quickening this memory of 100 years ago in the Cumberland Mountains. Don Baker, Frankie Taylor and Gary Slemp, in scores of roles, used mountain dialect so cannily that no one missed a word of passion, humor or narration. Individually gifted, the trio is marvelously resourceful as an ensemble.

This truly regional achievement, sponsored by Appalshop Inc., is a traveling theater rooted in the Central Appalachian coal fields of southwestern Virginia and eastern Kentucky. It once appeared at the Smithsonian Folklife Festival and would make an impressive attraction for Arena Stage, the Kentucky Center, Ford's or the White House East Room.

The critics' agenda included two new, full-length plays, reflecting a switch in the common thinking that the boondocks buy only titles made familiar through the national media.

The Nashville Circle Players' "The Wisteria Bush," by Jo Vander Voort, was selected from 24 entries is a contest sponsored by the Gannett Foundation. The story of five women in a small Alabama town, this was a competent exercise in a familiar vein. Played in the three-quarter space of the center's smallest theater, this pleased its audience through familiarity of its characters and situations.

The most surprising of Tennessee's companies surely must be Crossville's Cumberland Playhouse, not unlike Olney in size, with about 600 seats and an old-barn atmosphere. Founded 15 years ago by the late Paul Crabtree, who went from performing in "Oklahoma!" to writing and directing for the Theater Guild, this has made an impressive impact in its rural area with nine-month, partially Equity seasons.

Now managed by Crabtree's widow, actress Mary, and their eldest son, director James, the Cumberland's offering was a new play by Hal Corley suggested by his D.C. experiences as a government employe, "An Introduction to Paperwork Management."

Corley used 45 minutes to introduce a thread of narration, a weakness the play could not overcome. Office buffooneries were amusing in a mild way, but this was not enough. Had Corley begun his narrative line earlier and more firmly, one might have cared about his characters.

Another event was imported from Knoxville, whose Play Group performed a children's exercise in fancy -- a total delight. This splendidly resourceful ensemble, which also stages plays in the adult genre, uses the simplest props -- scruffy hats, far-gone tires, hoops -- with contagious imagination.

Nashville's children's theater is one of the most noted in the country and now the Academy Theater, in its 50th year, performs in the new Ann Stahlman Hill Theater. Its offering was Webster L. Small's first play for young people, a modern charmer. "The Boy Who Talked to Whales." This had welcome sparks of satire young audiences caught more quickly than their elders, and there is the solid advantage of children in children's parts, adults as adults.

College dramatics were represented by Vanderbilt University's lunch-time production of Edward Albee's "The Death of Bessie Smith" in its newly renovated Neely Auditorium.

The critics' association, which will be meeting in Washington next spring, didn't spend all its time watching plays. There was a panel discussion during which Louisville's Actors' Theater director Jon Jory distinguished himself by questioning how, if at all, the critics differentiate between acting and direction, thereby setting the theme of the next meeting.

Finally, the critics had the chance to discover Opryland, which seemed like a corny prospect but turned out to be an exciting training ground for young talents.

Opryland, nine miles from downtown, is unlike anything anywhere else, for its 110 acres embrace three distinct parts. Opryland Hotel, soon to be doubling its convention space and its rooms to 1,100, hints, with its elegant double staircase, that David O. Selznick stinted on his sets for "Gone With the Wind."

In 1975 Grand Ole Opry moved from the Ryman, where the earth's longest running radio broadcast began 57 years ago. Its new home, literally a broadcast-television studio, seats 4,000, and the Friday and Saturday night broadcasts, at $8 tops, are sold out as far as a year in advance. But it's as a theme park that Opryland is unique. Its themes are American music of all sorts and its performers are fresh-minted talents.

In nine areas scattered throughout the park, nearly 400 performers and instrumentalists appear in revues that run no longer than 45 minutes, their range from operetta and musical comedy to gospel, country, barbershop, rock and jazz. Since most of these are performed four to five times daily, there are alternate casts for almost all.

Despite the inevitable, ghastly hand-held mikes, one is impressed by the voices, grace and appeal of these young performers. John Haywood and his staff choose them from nearly 40 auditions scattered around the country, which cost management $100,000 to arrange yearly. Each audition may produce only two or three performers. But their promise and ambition are the strongest in their areas. They're paid $225 a week for from four months to a year.

The result is a remarkable training ground, where these young performers learn the disciplines and sweat of their craft, how to register instantly with an audience, as the old vaudevillians had to do. (Both King's Dominion and Busch Gardens have presentations like this on a smaller scale.)

In his producing job since Opryland's beginning, Haywood proudly notes that almost every Broadway and touring musical now has in it at least one Opryland vet. All but unobserved, then, a new outcropping of vaudeville exists here, the sort of place common folk relish and where tyro performers learn how to grab them. Any young entertainer would benefit from winning a spot in one of these productions, some of which, by the way, were written or staged by George Mallones, once of Arena Stage.

All told, the Nashville schedule exposed the conventioneers to a wild, wide range of previously unseen performers and audiences. There were the contrasts of subsidies and pure commercialism. Brooks Atkinson was right about Nashville as a learning experience for critics.