Some actors try to win us over by the sweat of their brows and the sheer agonizing labor that goes into their performances. They're the ones of whom it is said that they "throw themselves" into their art. And indeed, they look ready for a rest home at the curtain call.

Others enchant us by making it look so easy. They sail through the evening and appear positively refreshed when they come out to take a bow.

Rex Harrison and Claudette Colbert are sailers. There aren't many left these days and I'd advise you to catch them in "Aren't We All?," the vintage British drawing-room comedy that opened a four-week run last night in the Kennedy Center Opera House. The Frederick Lonsdale play is an utterly inconsequential, although not charmless, study of innocent dalliance among the upper classes. Give it a C. The stars are endearing. Give them an A. For effortlessness.

Actually, the stars belong to the subplot, but they thread their way through the proceedings with such ease and abundant good humor that you may be forgiven for thinking the play is really about them. Colbert plays a charming widow who has set her sights on Harrison, an aging but roguish lord, and if there's never a moment's doubt in our minds that she will hook her man, it's because there's not a scintilla of doubt in hers.

Now in her eighties, she is as handsome and graceful of face and figure as she always was. Her voice still ripples with mischievous musicality and her legs must be the envy of women half her age. (Gallantry, incidentally, is not prompting these observations; amazement is.) But that's only part of it. What makes Colbert so entrancing is her forthrightness. Her glamor is undeniable. But her attitude toward it is matter-of-fact. If we chose to look upon her as a phenomenon, she, frankly, will have none of it. She is merely going about the business of being Claudette Colbert, and let heads turn as they will.

It's perfectly understandable that at the conclusion of "Aren't We All?" Harrison agrees to matrimony without so much as a protest, although he does indulge in some delicious mumbling and an occasional whimper along the way. Age has treated Harrison somewhat differently -- endowing him with a paunch, a few additional chins and the squint of a sagacious eastern deity. But the irascible temperament has mellowed as well, and the old superiority in his eyes has given way to a new twinkle. Taking understatement to new heights, you might say, he makes comic acting look like a form of pure relaxation.

Seated side by side on a flowered sofa in the second act, the two engage in the subtle thrusts and feints of flirtation. The unpretentious scene constitutes the most appealing argument for senior citizenship yet devised by man or the Social Security Administration.

Meanwhile, it should be pointed out, "Aren't We All?" is busying itself with weightier (although only slightly) matters. At the very outset, Harrison's married son (Steven Sutherland) is lured into the arms of cool temptress. Just as he's kissing her, his wife (Lise Hilboldt) returns unexpectedly from a vacation in Egypt. Indignant, she promptly flies into a snit, even though it appears she may have dallied a bit herself under the desert moon. As the truth comes to light -- in the person of a dashing Australian (Ned Schmidtke) -- she will have to eat her righteousness, something Hilboldt does rather beguilingly.

You may have gathered that this is not an evening of broad delights. The fun comes from the arch of a fine brow; a subtle shift of tone; an ironic inflection that still manages to be polite; or the satyr-like jig that gives the lie to Harrison's statement that he has had "a life devoted to agriculture." There are no big chords, only grace notes.

What, then, is this play doing in the Opera House? Oh, I know. Making money. Still, if "Aren't We All?" isn't quite the supremely civilized triumph it was last season on Broadway, the fault lies not with the players. The hall is all wrong. Colbert and Harrison may be bigger than life. But from half of the Opera House seats they must nonetheless look about three feet tall. Nor can it be said that the amplification system works in favor of Lonsdale's gossamer dialogue. The actors toss off their lines and the loudspeakers promptly boom them out over the audience. A certain elegance is lost in the transmission.

Under Clifford Williams' direction, the cast members keep the frivolity as casual and intimate as the cavernous conditions permit. As the erring husband, Sutherland has the required looks (dapper) and just enough ruffle to his manner to suggest that he is truly repentant for his sins. Hilboldt's performance, initially chilly, warms up in the second act, appropriately enough as the hot water rises around her trim ankles. Schmidtke is dashing as the Australian swain, while George Ede and Joyce Worsley provide amusing counterpoint to so much high-bred style, as a stuffy vicar and his prudish wife.

Finlay James' sets -- a book-lined library in Mayfair, and a sun-washed living room in the country -- are as grand as the play is ephemeral. And the costumes will make you yearn for the days when people actually dressed up for breakfast.

Such assets notwithstanding, "Aren't We All?" remains Colbert and Harrison's property. At the end of the night, as they're sweeping out the French doors into the dappled garden, he casually lets it drop that he is 48. Not to be bested, she gleefully admits to 22. I believed them both implicitly.

Aren't We All?, by Frederick Lonsdale. Directed by Clifford Williams. Sets, Finlay James; costumes, Judith Bland; lighting, Natasha Katz. With Rex Harrison, Claudette Colbert, Lise Hilboldt, Steven Sutherland, Geroge Ede, Joyce Worsley, Ned Schmidtke. At the Opera House through Jan. 5.