By SANDY MACDONALD

In Love Child, writer/performers Daniel Jenkins and Robert Stanton make the versatile shape-shifters in 39 Steps - skilled as they are - look like pikers. With split-second timing (sometimes all they do to switch personae is pivot in place), Jenkins and Stanton play a score of comic theatrical types: the actors who are putting on a contemporized Greek tragedy in a sausage factory in Red Hook (including a hottie Mexicana and a haughty Old Vic poseuse who hails from Virginia), the roles they're meant to embody (the Delphic oracle in Euripides' Ion translates into a TV psychic), plus the family members who have come to kvell - and nudge. Among his many roles, Jenkins plays Joel, the anxiety-prone director, a resentful former child star- Stanton, similarly split-personalitied, portrays his pushy agent-mother.

We first meet the latter, Ethel, in the theatre's restroom, as she regales Kay, her semi-unhinged sister (Jenkins channels the shade of Alice Ghostley), with the tale of her very bad day. Deserted by a conniving assistant, Elissa, who decamped to a major agency with Ethel's client list, Ethel went to confront her and - in a fit of pique - ran out having pinched her Pucci purse. What do you want to bet that a ring tone of Fur Elise will interrupt the proceedings onstage? Which proceed none too smoothly, in any case, between the grease spot (a sausage by-product) that renders stage left unstable, and the handful of Klonopin that the post-hippie, Performing Garage-vet leading man (Stanton) lifted off frazzled Joel ...

A run-in with overly vocal audience kibitzers fifteen years ago inspired the two actors to concoct this side-splitting romp: that ordinary irritant has yielded an extraordinary pearl. The overlapping plots (Euripidean, backstage, "real" life) are so ornate and clever, the performances (under Carl Forsman's nimble direction) so appealing, that Love Child invites repeat viewings. Once your ribs recover, you may want to treat yourself again.