So conventional that its three tidy acts should probably be labeled "Beginning," "Middle" and "End," tonight's "American Playhouse" offering, "Breakfast With Les and Bess," seems a very endearing, not an irritating, anachronism. "Breakfast" is a play for old times' sake, set in 1961 before all hell broke loose and ran riot.

Dick Van Dyke and Cloris Leachman play a husband-and-wife radio team who broadcast a live program from their Central Park South apartment each weekday morning -- hence the title -- in the 1982 comedy by Lee Kalcheim that airs on the Maryland Public Television stations tonight at 9. No one who sees the play will be surprised to hear that Kalcheim has written lots of situation comedy scripts (but the better kind, like "All in the Family") or to learn that he first began working on "Les and Bess" 20 years ago.

"Funny, but trivial" is the way Leachman as Bess describes a movie the pair has seen, and the description fits Kalcheim's modest work, too, but as essentially superficial as the characters are, they do become real rather quickly. In part this can be traced to the poignant affability of Van Dyke, the aging cut-up -- a quality integral to the character of Les -- but it is Leachman's portrait of the frazzled Bess that gives the comedy not only bite but weight.

Leachman gets a surprising amount of affecting tension and soul out of this giddy woman whose idea of a fulfilling life is to sit around chatting into a microphone about last night's party or reading letters from refugee children, her pet charity. During the three consecutive mornings in which the play is set, events occur that shatter Bess' Steuben glass menagerie and clobber her with the realization that times are changing (though not a-changin'; that comes later). Leachman registers everything with either funny or touching, but always unmistakable, fidelity. She gives Kalcheim's work more respect than it deserves and turns it into something a little finer than was probably intended.

The couple's daughter, played with deft counterbalancing understatement by Wendel Meldrum, disputes the father's observation, late in the play, that Bess is a "strong" woman. "She's not a strong woman; she's a weak woman with a strong personality," the daughter says. The couple's son, a political activist playboy of some sort, is played by Shaun Cassidy, who has successfully weathered the transition from heartthrob to actor. He breezes through this with ripe aplomb.

Kalcheim got the idea for the play from actual husband-and-wife radio shows once popular on the air, especially one cohosted by columnist Dorothy Kilgallen and her husband, Richard Kollmar. There are references to Bess' appearances on a TV panel show; that would be "What's My Line?," on which Kilgallen was the master guesser. But the domestic travail Kalcheim invented was, he has said, just invention, and isn't meant to represent anybody's actual circumstances.

Les and Bess are certainly knee-deep in circumstances. Sonny-boy has driven his car into Central Park Lake as the play opens, and Les and Bess are trying to plop themselves before the waiting microphone. Then their daughter announces she has an announcement, but before she can make it, a naval ensign in his underwear is prowling about in the background. He, it develops, married the daughter the night before. Princess Grace is expected to phone any minute from Monaco, Les is dreaming aloud of returning to his previous career as a sports announcer, and according to the trade papers, talk shows like Les and Bess' are now first in line to become things of the past.

It all unfolds with a certain metronomic charm, kept clicking steadily by director Perry Rosemond, enhanced at every turn by the two engaging leads, demeaned by the kind of canned laughter that should never be allowed on public television. This play means nothing with about as much felicity as nothing can be meant, however. Not that it is without wisdom. When she learns that network executives have canceled the show, Bess snarls that they are "insensitive brutes," proving some things never change.

"Les and Bess" is certainly lighter fare -- even lighter-than-air fare -- than is usual on "American Playhouse," but then, it is pledge week, and we wouldn't want a serious play interrupted twice for gold digging, would we? It's easy to bemoan the drudgery of pledgerie, but a kind word should be said, too, for the way Washington broadcasting personality Paul Anthony handles this thankless chore on WETA. He's the best of the PBS panhandlers in this neck of the sticks by far, approaching the job with wit and vivant on the bon side.

Paul, if we must have our programs ruined and our pockets picked, you're the man to do it.