The success of rap records is no novelty; indeed, rapping for some time has been recognized as a major development in black music. But the basic rap format, in which an emcee improvises rhymes about his or her personal greatness over a repetitious rhythm track, has been pretty much exhausted in the years since "Rappers Delight." The rappers' problem: keeping it fresh.

The solution offered by "King of Rock" (Profile PRO-1205), Run-DMC's second album, has more to do with the music than with the rapping.

The title cut, for example, matches Run-DMC's tough-talking word play with a brutal combination of drums and heavy guitar. Run-DMC used this strategy before, on last year's ground-breaking single "Rock Box," but because the monolithic riff pounded out by guitarist Bettie Martinez could as easily have fit on a Black Sabbath record, the fusion of hip-hop and heavy metal sounds amazingly natural, as it ought -- the type of rapping practiced by Run-DMC shares a number of characteristics with heavy metal. Both rap and metal emphasize the aggression implicit in stomping out a single riff for the length of a song; both forms celebrate the self as a triumph over oppression; both define themselves in terms of a mass, unified audience. That's not to say that Run-DMC is the bridge between rap and heavy metal audiences, but the possibility is definitely there.

Although guitars feature heavily on four of the album's nine cuts, there are plenty of other interesting musical devices on hand. Some, such as the reggae infusion in "Roots, Rap, Reggae," recorded with Yellowman, are fairly obvious, but others are remarkably inspired. "Rock the House" starts off the album with a breathtaking sonic collage mixed by the Latin Rascals. Acting almost as an overture, it drops bits of other raps from the album atop a lean percussion track, chopping the phrases until they're reduced almost to pure rhythm. "It's Not Funny" plies a similar mix of sound and sense as Jam-Master Jay backs the hard-time lines with a scratch-mixed groove built from an angry "ha-ha -- yeah, funny."

Still, it's the verbal wit of Run (Joe Simmons) and DMC (Daryll McDaniels) that pulls the album together. Even their most outrageous rhymes, like DMC's boast, "I'm the king of rock, there is none higher/Sucker MC's should call me sire," are delivered with such undeniable flair that it's hard not to take them at face value. True, "King of Rock" doesn't quite deliver any commentary as bitterly appropriate as on the group's first single, "It's Like That," but the level of expression attained in "You Talk Too Much" and "It's Not Funny" bespeaks enormous growth from the two rappers.

It's worth remembering, too, that merely dressing up the rhythm tracks is not enough to make a rap record worthwhile. Certainly that's the message delivered by "They Said It Couldn't Be Done" (Elektra 60389-1), the latest from Grandmaster Flash.

Grandmaster Flash throws all sorts of musical polish into his grooves, from the scratching he pioneered to elaborate horn arrangements, but instead of enlivening the beat, it enervates it. The rhythm tracks are just too busy, almost as if Flash and crew were overcompensating for a lack of inspiration.

Not that they don't have reason to overcompensate. After the Furious Five broke up last year, with Grandmaster Melle Mel, Scorpio and Cowboy heading off on their own, Grandmaster Flash was hard pressed to prove that he was leader of the band in more than name alone. Unfortunately, the raps concocted by Kid Creole and Rahiem barely approach the polysyllabic panache of Melle Mel's routines, while their singing makes their rapping skills seem monumental by comparison. "They Said It Couldn't Be Done"?They were right.

At the moment, though, rap's most successful spinoffs have nothing to do with the Furious Five -- they're the "Roxanne" singles. The first, U.T.F.O.'s "Roxanne, Roxanne" (Select FMS 62254), was a wry and witty story of how the three members of U.T.F.O. struck out while trying to woo the lovely Roxanne. Not only was this an amusing twist on the fly-guy approach most male rappers take, but U.T.F.O. added innovative twists to their rhymes.

That single became so successful that a rival rapper, Roxanne Shante, turned up with an answer record, "Roxanne's Revenge" (Pop Art PA-1406). With a voice barely more than a squawk and a delivery that was at once randy and innocent, she made "Roxanne's Revenge" a better street record than U.T.F.O.'s original. Of course, that street version wasn't entirely suited for air play, so the 12-inch single adds a better-produced, cleaner version as a B-side.

Nonetheless, U.T.F.O. weren't about to be upstaged; they rushed out their own answer record, "The Real Roxanne" (Select FMS 62256). In its way, this record is an even more devastating put-down than Roxanne Shante's revenge; at one point, the real Roxanne describes the group's girlfriends as "bowwow babies" adding, "I think they got rabies." As rankfests go, this is unbeatable.

Still, the funniest rap record on the radio right now has got to be "Rappin' Duke" (JWP 1456). Performed by an unnamed John Wayne impersonator, it takes the premise that the Duke's real calling was rapping to the beat, and proceeds to an illogical, and utterly laughable, conclusion.