A.R. Gurney Jr.'s "The Dining Room" is one of the most civilized new comedies to play the Eisenhower Theater in seasons. If that smacks of vaguely starchy praise, rest assured: Theater The work, which observes the traffic in and out of a formal dining room over several decades, is never less than abundantly entertaining.

What Gurney is specifically chronicling--in the 20 or so overlapping scenes that jump backward and forward in time--is the evolution in manners and mores of the WASP culture. But anyone who remembers being told to sit up straight, napkin on the lap, please, or has briefly wondered just what is the proper approach to a finger bowl, or has known both the discomfiture and the warmth of holiday get-togethers around the ancestral dining table will respond to Gurney's insights.

Already a substantial success off-Broadway, there is every reason to believe "The Dining Room" will duplicate its popularity at the Kennedy Center, where it officially began a six-week run Saturday night. The fine six-member cast is headed by Frances Sternhagen, an actress who can make common sense the most enchanting of qualities, and Barry Nelson, an actor who remains engagingly boyish when boyishness in others his age has become merely boorish. David Trainer, a director with a canny instinct for the revealing detail, has recreated his original staging, and it is sociologically and dramatically flawless. Housed here in considerably larger quarters than it is in New York, Gurney's play still ushers an audience into its special confidence.

The six actors are actually responsible for about a dozen roles apiece, ranging from unruly tots to crotchety grandparents, with a fair cross-section of upper-middle class WASPs in between, not to mention the procession of maids, who push back and forth through the swinging door to the kitchen. Each scene is little more than a vignette, but those vignettes, like well-placed windows, open onto a whole society.

A young man tries to convince his independent grandfather to spring for tuition to a posh boarding school. A mother is caught in the middle of an afternoon fling by her son, returning unexpectedly from college. A Thanksgiving dinner goes awry, when a senile grandmother asks suddenly to be taken home, although that is precisely where she is. In one scene, ineffably touching, a son fights to keep his emotions under control, while his father explains, in details both noble and insignificant, the kind of funeral he wants. In another, rueful and plaintive, a daughter, who has made a botch of her life, tries to come back to the nest, only to find that there's no nest left. But Gurney can be robustly comic, as well, and never so much as when his characters sniff a slight--to their honor, their sense of decorum, or even English grammar--and feel compelled to set it straight.

There is an underlying order to the various snippets, however, even though a scene in 1950 may be succeeded by one 20 years later or 10 years earlier. A way of life is on the wane, and the stern belief in duty and respectability that was once the backbone of WASP culture is going with it. Midway through act two, a brash college student gets his aunt to haul out her bone china, the Irish linen placemats, and the Steuben waterglasses, then carefully explain their use, while he snaps away with a camera. When she asks him why, he says it's for a term paper at Amherst on the eating habits of "vanishing cultures."

Bit by bit, the dining room itself--as much a character in Gurney's play as any--is being phased out. Modern life styles make it an archaic and cumbersome gathering place. Servants get harder to come by. Even the table (not valuable or antique, "just American") that dominates the Eisenhower set develops its creaks and wobbles. Gurney is all too clear-headed a playwright to settle for easy nostalgia, though. He threads a very deft path between mockery and admiration for the creatures in his tight WASP world. Some of what is being lost--discretion, self-reliance, table manners--is valuable. But there are also empty rules, stale customs and a preoccupation with appearances that deserve to go. "When my sister had anorexia," whines one disgruntled teen-ager turning her exasperation on the table itself, "she still had to sit here and watch, for God's sake." As a result, the production is often likely to catch you somewhere between laughter and tears.

The economy of Gurney's writing is matched by the cast, which gets to the heart of each passing crisis in jig time. The actors pop up in many guises, but Mary Catherine Wright's deadpan serves her splendidly, as senile relative or determined servant. Richard Backus can sit stock still, as a teen-ager on the spot, and give the impression he is squirming like a can of worms. Jeanne Ruskin's suburban ladies have grace under pressure, even when they're being disloyal to their husbands. Peter Coffield's slick, preppy looks provide a neat counterpoint to the ragged turmoil within, needless to say, the archetypal WASP dichotomy. And no small pleasure comes from seeing Sternhagen and Nelson, the elder members of the troupe, exit as responsible adults only to reappear seconds later as protesting kids. Or vice versa.

The future may condemn us all to modernistic "eating areas." After all, folding laundry is what we now do on the dining-room table, assuming we even have one. But I suspect that Gurney has captured part of everyone's past--WASP or not. "This room has such resonance," marvels a shrink, who finds himself wavering in his plans to convert it into an office. So does this intelligent and ingratiating play.