"Romantic Comedy," a descendant of the high-style plays that flourished in the 1930s, made its bow last week at the Ethel Barrymore theater. It will be flourishing well into the 1980s, stylishly crafted, wittily amusing.

The setting is an elegant New York townhouse. The author is Bernard Slade, who cooked up "Same Time, Next Year," and the brightest of six bright performers is Mia Farrow, who is an instant delight in her Broadway debut.

Her role is the kind actresses murder for, the hick who turns into a worldling in seven scenes covering 13 years. Though many will play it from here to Hong Kong, Farrow will be remembered as its creator.

Existing this last decade largely through a few films and gossip from abroad, Farrow has been honing her stagecraft in England's Royal Shakespeare Company, where she acted a variety of major roles. That broad, high-cheekboned face that suffered so in "Rosemary's Baby" and "The Great Gatsby" is accompained by the most priceless of stage qualities -- a clear, strong, finely articulated voice. Phoebe Craddock, her character, is endearing, outspoken, witty and womanly, and Farrow sparks all these tones and many more.

A twentyish, prim Vermont third-grade teacher, Phoebe has come to New York to see how Jason, an idolized playwright twice her age, has received her unsolicited manuscript play. Knowing he needs a new collaborator, Jason's agent has recommended he consider Phoebe.

Jason has just sniffed bitchily into the phone to his defected male collaborator that he "is having a very busy day. I'm having a massage and I'm getting married." While Phoebe waits for him, Jason backs into his study ripe for his massage. Phoebe is stage center, face front, and watching her eyes dart for anything else to look at except the naked Jason is our first glimpse of Slade's visual fun.

Allison, Jason's bride, enters in white. Their wedding guests are restless but she is equal to merely another eccentricity of her genius-groom. Jason will use both relationships -- his ready bride and the new collaborator he so badly needs. Allison will be delighted to have an experienced governess on tap.

Deftly acted by Anthony Perkins, whose father, Osgood, scored in like parts in the 1930s, Jason is an insider's view of Genus Playwright -- insecure, egoistic, bitterly sharp and vulnerable to his need for any help.

Under the superficial sheen lies a credible study of contemporary relationships: the man who snaps, " married Grace Kelly and she turned into Bella Abzug"; the wife for whom neglect opened up a public career.

Slade also exudes stylish Cowardish lines: "Victims of unsynchronized passion," "Just because he's never had a commercial success doesn't mean he's talented." I'd have welcomed a slightly tighter final five minutes, but Slade's mirror-of-life imagery is never oppressive.

Holly Palance, Jack's daughter, is deliciously in command as Allison; and Carole Cook, in an array of billowing outfits, is the essence of acidity as the informative agent. Greg Mullavey, as the earthy correspondent who whisks Phoebe away, is exactly right; and Deborah May is wildly funny as an actress who is used to others dressing her. Director Joseph Hardy's stylish restraint fits precisely.

Producer Morton Gottlieb, who has scored his greatest hits with two-character plays such as "Sleuth" and "Same Time, Next Year," here employs a glossy cast thrice their demands and such elegantly accomplished talents as designer Douglas W. Schmidt, costumer Jane Greenwood and lighting magician Tharon Musser.

"Romantic Comedy" is Broadway's first non-musical hit of the season, and what a welcome pleasure it is!