It's no coincidence that as the Reagan regime was polarizing the country along racial lines in the early '80s, commercial radio began systematically dividing its demographic pie in the same way. Pop music's family tree split down the middle, seemingly for good. But now two of that tree's far-flung branches – rock and rap – are joining up again. Imagine the possibilities. Rap-rock fusion is nothing new, of course – the very earliest rappers dropped in records such as Billy Squier's "Big Beat." And the big beat is what unites all the groups on the mighty soundtrack to Judgment Night, which features 11 full-fledged collaborations between hip-hop musicians and rock bands, most of them from the more metallic edge of so-called alternative rock. Cypress Hill mix it up with Pearl Jam, Helmet kick with House of Pain, Ice-T throws down with Slayer.

Long before Public Enemy and Anthrax united for a version of P.E.'s "Bring the Noise" in 1991, Run-D.M.C. unveiled the pioneering "Rock Box" (1984) and their landmark collaboration with Aerosmith on "Walk This Way" (1986). In 1986 as well, the Beastie Boys sampled Led Zeppelin on Licensed to Ill and later went on to bring rap rock to some sort of apotheosis on last year's brilliant Check Your Head. This year, the apoplectic rap group Onyx exhort fans to "slam" on their latest single, rap auteurs the Butcher Brothers (Schoolly D, Cypress Hill) produced the new Urge Overkill album and Lollapaloozers dug both Arrested Development and Alice in Chains. The two camps even wear the same baggy pants and baseball caps.

Rap rock rises as the two most innovative pop-music forms eye creative burnout and seize the opportunity to improve the stock with a hybrid. The two genres dip from a common well of alienation, hence a teaming like rapper (and leader of his own hardcore band, Body Count) Ice-T and the apocalyptic thrash-metal outfit Slayer (who roar, "We don't want your war," even though their music was a virtual soundtrack for Operation Desert Storm). For white kids, it's now more rebellious than ever to embrace black culture. Likewise, resourceful rappers are looking for new sounds, a hard edge and crossover appeal; after all, whites purchase the majority of rap records. And rappers are more musically broad-minded than antirappers might think – how do you think they find all those cool samples?

Oh, to have been a fly on the wall at these sessions. "Me, Myself and My Microphone" is a crackerjack Gotham teaming of Living Colour and Run-D.M.C., both forefathers of rap rock; on that track a deft sample of Run-D.M.C.'s own "Hollis Crew" punctuates a pulverizing metal riff. Scottish college-rock darlings Teenage Fanclub and De La Soul ride one simple, lazy, hazy groove to summer-pop perfection on "Fallin'," a poignant tale of a washed up rapper who's lost his muse. On "Just Another Victim," Helmet lead barker Page Hamilton and House of Pain rapper Everlast take their own fervid approaches to the same sheet-metal guitar onslaught, riddled with nasty percussion samples.

Over the slammin' drum loop of "Another Body Murdered," Faith No More subvert the bravado of Samoan rappers Boo-Yaa T.R.I.B.E. with a doomy piano figure, bloodcurdling screams and a corny horror-flick vocal line. Another odd couple, Del tha Funky Homosapien and Dinosaur Jr, collaborate on "Missing Link." Del raps over a loose-limbed funk line, while Dinosaur Jr's J Mascis cuts in a croaky vocal obbligato, dollops of wah-wah and a guitar solo that aptly quotes "Purple Haze." One of the best cuts comes from the unlikeliest pairing: proto-grunge rockers Mudhoney and their pygo-philic Seattle homeperson Sir Mix-a-Lot. "Freak Momma" is a gleeful, rockin' slice of what can only be described as garage rap.

Riding a lopsided groove that rolls along like an unbalanced tire, Sonic Youth and Cypress Hill team for "I Love You Mary Jane," an ode to America's latest retro drug craze. Except for an incantatory Kim Gordon vocal hook and a little guitar art around the edges, the Hill apparently ruled the tune. Not so for "Real Thing," the tokin' rappers' thoroughly on-the-one collaboration with Pearl Jam, which closes the album on a visceral note of rock abandon and rap attack.

In its own inbred, fifth-generation way, Judgment Night's bracing rap rock is like the wedding of hillbilly and "race" music that started the whole thing in the first place. It may not be as culturally momentous as the very birth of rock & roll, but it's an inspiring rebirth, and these days, you take what you can get. (RS 666)

MICHAEL AZERRAD