THEY’VE seen it all in “Hair” and heard it all in “The Vagina Monologues.”

But will audiences flock to a Broadway musical that hangs on the question, “To pee or not to pee?”

We’ll soon find out when “Urinetown: The Musical” opens on the Great White Way.

The quirky show started at the New York International Fringe Festival a couple of summers ago, then opened this spring in the American Theater of Actors, a dumpy, tiny off-off-Broadway theater no one ever heard of.

Propelled by rave reviews, “Urinetown” is trickling along the path paved by “Rent” to the seediest Broadway playhouse available, the all-but-derelict Henry Miller.

But Broadway it is, and a top-flight team has put its formidable weight behind “Urinetown” – from the producers of “The Vagina Monologues” and “The Dinner Party,” to Edward Strauss, musical director of hits such as “A Funny Thing Happened on the Way to the Forum” and “Guys and Dolls.” Even John Cullum, the two-time Tony winner, stars in “Urinetown.”Now all it needs is an audience that wants to pay $75 to see a self-conscious musical parody about a mythical place where it’s no longer free to pee.

In London late last summer, I found myself sitting next to Cullum at the National Theater’s production of Arthur Miller’s “All My Sons.”

I asked him what he was going to do next. He smiled mysteriously and said: “It might be a surprise.” Boy was it ever.

In “Urinetown,” he plays Machiavellian villain Caldwell B. Cladwell, who in times of ecological disaster and its subsequent drought takes over the town’s water.

His corporation also bans private bathrooms, institutes the instant disappearance of anyone caught informally relieving themselves alfresco, and charges a prohibitive price for the use of the town’s public toilet facilities.

There are lots of big song-and-dance numbers wrapped around a love story, a tragedy, a people’s revolution and an adorable little street urchin who falls in love with the hunky leader.It’s a mixture of sophistication and naivete, of down-at-the-heel glamour and Broadway chic, and it won the startled hearts of virtually every critic who saw it.

By opening it in a ludicrously small theater in Hell’s Kitchen – a 131-seat ad hoc auditorium in the Midtown Community Courthouse – “Urinetown” became an even hotter-than-ever ticket, with a strong must-see word-of-mouth.

The show closes tonight and will reopen Sept. 10, after a month of previews at Henry Miller Theatre, a small Broadway house with only 500 seats.None of this makes “Urinetown” a surefire hit.

Only a couple of seasons ago, a little off-Broadway show called “Squonk,” emboldened by an off-Broadway rave from the New York Times, squawked onto Broadway. Well, the Times didn’t find the “small, eccentric charmer” so cute in that big theater, calling it “a brutally misguided transplant.” The show was promptly squelched.

The “Urinetown” folks will certainly have to spruce up the bleak set – a stained brick wall representing a municipal comfort station.

But I would bet on its success. It’s a wonderful show, adroitly marketed to maintain its sleeper reputation, even though it’s loaded with established top talent.

Toilet humor is no longer frowned upon. And it seems only yesterday that dear old Eliza Doolittle was shocking the patrons of Shaw’s “Pygmalion” with her simple use of the dreaded curse word “bloody!”

Are we ready for “Urinetown”? I say, let it rip. Or is it drip?