In the theater's vast storehouse of goods, there are "kitchen sink dramas," "drawing room comedies," and even "closet dramas." Paul Osborn's "Morning's at Seven," which opened a three-week run at the National Theatre last night, is a rarer breed--"the back-porch drama."

That's where this gentle and slightly daft account of four elderly sisters and their assorted relatives takes place. Actually, Osborn calls for two back porches and the trim patch of lawn between them to contain the hurt feelings and the momentary misunderstandings that crop up in the course of a midwestern night and the morning after. But it's all the same. What we're really talking about is a perspective.

The front porch is where a family puts on airs, minds its manners and announces its respectable standing in the community. The back porch is where it lets down its hair, gives in to displays of emotion and, with the stars twinkling overhead, sometimes ponders this merry jest called life. The back porch is a place for exchanging gossip that isn't quite respectable or necessarily very true. It's also the place you stomp off to, when you want to blow off some steam. Or when you want someone else to come running after you.

Osborn's characters, lovingly and deftly drawn, do all of the above and more. And while, in the great scheme of the universe, their tiffs and squabbles don't really amount to a hill of beans, their passions are so authentic--and so authentically rendered by a fine cast--that you will want to hug them all to your breast by the evening's end. A failure when it was first produced in New York in 1939, "Morning's at Seven" only revealed the full measure of its charms two years ago in a splendid Broadway revival, meticulously directed by Vivian Matalon. About half of the cast members at the National Theatre are veterans of that production and the newcomers have done nothing to disrupt the homely texture of the play.

If Osborn didn't take such a quietly philosophic approach to his characters, things could get rather bloody in "Morning's at Seven." Consider the sundry plights of his four sisters. Ida (Kate Reid) has a husband with "spells," which is to say he goes bonkers periodically, leans his forehead against a tree in the yard and contemplates the fork in destiny's road he didn't take. She also has an excruciatingly timid son (Robert Moberly), who after 12 years of courtship has finally mustered the courage to bring home his fiance'.

Just a patch of green grass away, Cora (Teresa Wright) has been sharing her house for 45 years with a third sister, Aaronetta (Elizabeth Wilson), a pinched old maid who wears her spinsterdom like a hair shirt. If back-fence gossip is correct, Cora's also been sharing her husband, as well. Further down the road is Esther (Maureen O'Sullivan), married to an intellectual snob who views the entire family as a clan of morons and has threatened to banish his wife to the second floor of their abode if she so much as frequents her kin on the sly.

Jealous husbands, possessive mothers, banishment, madness and illicit sex. This is the stuff of Greek tragedy. Osborn, however, has set his play in America in 1922 and, optimist that he is, is not about to get too het up over events. He knows that every family closet has a skeleton or two and is no worse for that. He also knows that if molehills can be made into mountains, mountains can--with forbearance, common sense and a little understanding--just as easily be turned right back into molehills. His characters may have a few screws loose, but their sense of decency is nailed down tight.

As a result, "Morning's at Seven" has the humble glow of fireflies in the garden and the warmth of laughter bestowed on dear friends. And its virtues are brought to a high polish by an accomplished cast that doesn't seem to be acting this drama as much as living it. Wilson's spinster is the plum, entirely too vaporous a martyr to do much harm, but persistent enough in her self-dramatizing ways to be richly entertaining. O'Sullivan projects a silvery grace and a sweet flirtatiousness. Wright is soundly pragmatic. And if a red-faced Reid finds herself playing the grasping mother occasionally, she's quite as surprised as anyone else when it's pointed out to her. Although the four sisters have long since passed 60, the actresses are very clever about showing us traces of the girls they once were.

The husbands--Maurice Copeland, King Donovan and Russell Nype--have been equally well chosen. Donovan, especially, orchestrates his tics and spasms to suggest that Ida's mate is not unlike an overwound cuckoo clock, springing its gears. You'll notice some of the erratic gestures neatly echoed in the keen performance of Moberly, as the timid son, who in his 40th year finally flies from the nest with an awkward flapping of wings.

As he flaps away, Myrtle (Charlotte Moore), his perfectly plain fiance', beams at all her new relatives and proclaims them just about the nicest people God ever put on Earth. Moore understands the eager forthrightness of the wallflower who's grateful to be a bride. But if her enthusiasm is comic, it's also entirely justified. The back porch folk of "Morning's at Seven" may occasionally be petty and misguided. But their hearts are good and full.

And so is the play.

MORNING'S AT SEVEN. By Paul Osborn. Directed by Vivian Matalon. Sets, William Ritman, costumes, Linda Fisher, lighting, Richard Nelson. With Maureen O'Sullivan, Kate Reid, Elizabeth Wilson, Terese Wright, Maurice Copeland, King Donovan, Robert Moberly, Charlotte Moore, Russel Nype. At the National Theatre through March 14.