The two women are revealed perched high above the stage, looking down at the audience from behind a door in the single wall that serves as the set. As they discuss the creation of the human race - everything from the assignments of multiple skin colors and sexual orientations to the roles both sexes will play in child rearing - it seems as if the women are re-enacting the evolution of humor itself.

In a way, they are - their unique brand of humor, at least. This is about as angelic as the women get, though; the rest of the time, they have to settle for being merely wholly entertaining. Not that it matters - with a couple of minor exceptions, just about everything Kathy Najimy and Mo Gaffney do in their new comedy show Afterbirth: Kathy and Mo's Greatest Hits, which is playing at Second Stage through July 11, feels as fresh as can be.

That's a particularly impressive achievement since, with the exception of a few updated references sprinkled throughout the two-hour show, only two of the sketches are new. The rest were taken from the duo's previous successful (and award-winning) stage and television shows, and may be familiar to those who've followed these women's careers from their more humble beginnings to the movie and television fame they've now achieved.

It's perhaps a bit unsurprising that the new sketches feel less fully formed, less comically substantial than the more familiar ones, many of which can legitimately be classified as "greatest hits." But with neither of the new entries - about a support group for the often absent mothers of Disney heroes and heroines, or for two middle-aged women facing the challenges of growing old gracefully - is there the sense of incompleteness or disappointment. The ideas are solid, but haven't yet been polished in quite the way the show's other pieces have.

That makes sense, in a way - the women are fairly new to the feelings those pieces express, feelings more akin to relatively new mothers in their 40s (as both Najimy and Gaffney are) than the younger women that first started working together well over a decade ago. But the other pieces, generally about more timeless subjects and troubles, have been so brilliantly honed and refined that they contribute greatly to making Afterbirth the stratospheric fun it almost always is.

It's not just that the material is ingrained in the women's muscles, bones, and voices, though that's clearly true. It's that the connection between Najimy and Gaffney is so solid - they play off of each other beautifully, picking up on each other's thoughts and comic impulses as though they were two halves of one person. So, while the women do get chances to shine in monologues (Gaffney in one about abortion, Najimy in a particularly touching one about a young relative's battle with AIDS), their best work is always done together.

Director Mark Brokaw doesn't let anything interfere with that relationship - his staging is minimal, and he lets their palpable natural chemistry carry just about every scene. The set, costume, and lighting designs (by Allen Moyer, Linda Ross, and Mary Louise Geiger respectively) are usually suggestive at best, and rely on Najimy and Gaffney to establish every locale with intricate emotional, verbal, and physical detail.

Neither actress ever disappoints. Whether playing young teenagers testing the limits of their friendship in the all-too-important ways youngsters do, suburban matrons on a women's studies field trip, the performers in a rollicking feminist performance art piece, or even exaggerated versions of themselves, Najimy and Gaffney bear the kind of electric comic presence that unequivocally identifies them as the stars they are. The audience response greeting the opening of the penultimate scene - in which the very southern Hank (Najimy) and Karen Sue (Gaffney) unload their most precious emotional needs and desires (buried within mountains of humor, of course) - suggests that these women are simply close family friends who've been away for too long.

All of Afterbirth feels just that way, and while it's primarily a comedy show, it's got enough dramatic heft to seem perfectly at home on just about any stage you can name. Let's hope that if Najimy and Gaffney aren't willing to make the theatre district their permanent home, they'll at least make it a more frequent vacation spot.

Afterbirth: Kathy and Mo's Greatest Hits
Through July 11
Running Time: 2 hours, with one 15-minute intermission
Second Stage Theatre, 307 West 43rd Street, between Eighth and Ninth Avenues
Schedule and Tickets: 212-246-4422