LIKE THE PROVERBIAL, little girl who, when she was good was very very good, and when she was had was horrid. Sam Shepard's style spans an unusually wide range of quality. Unlike the little girl, though. Shepard has evolved out of the bad into the good, and if his latest play. True West, is any indicator, he will not return from whence he came.

Shepard's earlier works are weakened by attempts to universalize uninteresting and generally ludicrous particulars. The characters are two-dimensional, artificial constructions; their conversations are heavy-handed and foolish. Shepard's early tendency toward bald, over stated symbolism has mellowed to produce the subtle, relevant statements of his most recent works, Buried Child and True West.

True West possesses all the qualities that make Sam Shepard one of America's greatest living dramatists--biting humor, pungently realistic characters, emotional appeal, and an arresting, poignantly developed central conflict. The play treats the struggle between civilization and primitivism; more specifically, between an Ivy League-educated Hollywood screenwriter and his brother, a near-illiterate renegade who has just emerged from five months of solitary foraging in the Mojave Desert. Thrown together in their mother's house--she is vacationing in Alaska and her well-behaved son is baby-sitting her plants--the do-good and the no-good brothers clash, switch roles, and clash again.

From the first scene the split between the two is clear. Defending his childhood predilection for make-believe, good-boy Austin (Francois de la Giroday) claims. "I enjoy my imagination "Lee (John Bottoms), recalling his own boyhood love of squashing desert snakes and disgusted by his brother's lack of macho, snorts. "Looks like you're still enjoying it."

At the same time, Lee is desperately envious of his brother's success, of his home and car and wife and children, of his solid bourgeois existence. Sneeringly, but also wistfully. Lee refers to "houses like they have in magazines. You know, with blondes moving in and out of rooms. "Lee's ambivalence toward security and society crystallizes when Austin invites a movie producer. Saul Kimmer (Richard Grusin), to the house to talk business. Scoffing openly at Kimmer's lifestyle. Lee the dirty, ill-spoken, scowling failure babbles with gleeful sarcasm about his imaginary residence in Palm Springs, his love of gold, his familiarity with Hollywood's Bob Hope Drive. But when Kimmer indicates interest in Lee's idea for a Western, the misanthropic scoundrel becomes bell bent on scoring a success in the movie world.

As Lee and Kimmer negotiate a deal on the Western. Austin--whose own script has been junked to make way for Lee's--falls apart. He lies under the kitchen table swilling Jack Daniels, changes his pressed khakis and navy pullover for a sweaty. T-shirt and jeans, and finally gets out to prove himself on his brother's own terms Austin the Eastern-educated, the well-dressed, the content and successful, bets that he can steal a toaster.

And he does. Austin's larcenous achievement serves as the vehicle for a favorite Shepard effect: the symbolic over-abundance of food-stuffs. In Buried Child vegetables from the backyard garden were heaped on the stage in Rubens-esque quantities. True West serves up toast made in the five toasters Austin steals. These masses of toast represent Austin's hostile offering of his own success in his brother's line of work.

THE PLAY's title provides a key to the symbolism behind the fraternal conflict. Lee represents the true West, the land of cowboys and Indians and rugged individualism--which, according to Austin, has been entirely replaced by freeways and high-rises and Hollywood operatives. Austin cannot understand Kimmer's enthusiasm about Lee' movie, because it is a real holster-grabbing, double-barreled Western "I m the one who's in touch, not him?" Austin yells at Kimmer But the movie producer doesn't agree: he thinks Lee's plot has honest-to-goodness grit. "We make movies. American movies. Leave films to the French," he snaps at Austin. The criticism is implicit but clear: the West survives because the American people want it to survive, because it cannot be expunged from our hearts. Austin is rooting for the wrong side.

His misplaced allegiances are not without justification, though. By denying his links with the West. Austin is seeking also to escape his and Lee's parents, who represent a warped flip side of the American dream. Their father, who remains offstage, is a drunkard who once put his false teeth in a doggie bag filled with chow mein, and then left the bag in a bar along a Mexican highway. Their mother, who returns from Alaska in the final scene, is perfectly tacky, uncomprehending, and well-meaning. She appears terribly excited because she read in a newspaper that Picasso is visiting the local museum. She refuses to believe Austin's claim that Picasso has been dead for years and instead tells Lee, who has never heard of Picasso, that they will go that afternoon to meet the artist. Ignoring their mother's chatter. Lee and Austin square off for the ultimate battle in the house they have torn apart. And as each savagely attempts to kill the other, their mother says Imploringly. "Now, boys, you don't have to fight in the house. There's plenty of room outside to fight."

John Bottoms and Francois de la Giroday deliver arresting, finely tuned performances. De la Giroday's sardonic antics with the mounds of toast his drunken, bitter humor, and his ability to shift gears--to portray both a self-possessed success and a collapsed failure--are outstanding. Bottoms' stooped, hulking gait and his combination of down-dirty badness and querulous insecurity breathe life into a difficult and confusing character. He recalls Henry Fonda at his most cranky in On Golden Pond in his ornery refusal to admit that he is pleased by something done for him, his obstinate pessimism, his scorn and his inability to be simply friendly.

In smaller roles, Richard Grusin and Shirley Wilber also deliver telling, pungent performances. Seen most recently as Zoditch in the A.R.T. production of Journey of the Fifth Horse. Grusin demonstrates here his ability to play a balding, affected, overweight Hollywood producer as well as a sour old reader in a 19th-century Russian publishing house. As the mother, Wilbur is appropriately fussy and matronly: Her high nasal whine sounds very good.

The technical aspects of the production are also uniformly excellent. Nancy Thun's costumes--particularly Kimmer's white leisure suit and Elton John-style glasses--contribute to the southern Californian atmosphere without smothering the play in stereotypes. The music between scenes provides some fine, although not classic, examples of old-style, sloppy, thumping, twanging country-western blues.

Shepard at his best is galvanizing. It is impossible not to be affected by the conflict between Lee and Austin. But True West provides more than good dramatic tension, because its symbols endure. After years of trying, of registering heavy-handed, ponderous failures, Shepard has achieved a style that hits its mark. The West may be a thing of the past, or--as Shepard believes--it may endure because of the irrepressible nature of human beings. Whether or not the days of sheriffs and gunfights are over, Shepard himself wields a mean gun. His range is long, his sights are excellent, and his shots are right on target.