From Little Richard to the Rolling Stones, sex has been the central subject of much rock 'n' roll. Nevertheless, pop music has long avoided film's headlong plunge into quickly boring graphic details. Rock singers always have preferred the humor and poetry of double entendres, metaphors and euphemisms. Spelling everything out might diminish the listener's imagination and enjoyment.

Now, however, two of pop's most gifted artists -- Marvin Gaye and Prince -- have gambled that they can sing uninhibitedly about sex and still retain the mystery of their music. They succeed because their music is rich enough to maintain an aura of mystery around the most explicit description. On "Midnight Love" (Columbia FC 38197), Gaye's erotic pillow talk takes its mystery from the expansive, after-hours music that seems to swell with myriad rhythmic, harmonic and sexual possibilities. On "1999" (Warner Bros. 23720-1F), Prince's frank physical challenges take their mystery from the bristling, impatient funk that moves and turns so quickly that anything might happen at the next moment.

Interestingly, both albums are very private efforts. Both Gaye and Prince wrote, arranged, produced and performed the albums almost entirely by themselves. Except for some guitar and horn parts on "Midnight Love" and some background vocals on "1999," Gaye and Prince played every instrument and sang every vocal. This was made possible largely by their innovative use of synthesizers to create bright horn charts, thundering dance tracks and harmonic sheets of sound. This private music-making made possible the perfect erotic fantasy and produced an insularity from the rest of life that ultimately weakens both albums and makes them less compelling than the artists' best work.

Gaye pioneered sexual explicitness with his landmark 1973 album, "Let's Get It On." He has now returned to the themes and sound of that album and taken them a step further. His new hit single, "Sexual Healing," finds the musical equivalent for this doctoring in a rhythm track that undulates with the competing rhythms of American soul and Jamaican reggae, and a vocal that cruises with calm. "Rockin' After Midnight" takes an up-tempo approach with Joel Peskin's saxophone and Gaye's voice emitting squeals of joy.

Throughout the album, Gaye uses drum machines, trap drums, bells and congas to give the rhythm tracks an unusual diversity. He once again uses the stereo spatial mix to create the illusion of immense space in each song. These studio techniques -- coupled with languid, sighing synthesizers -- create a private bedroom where the listener is ushered in to hear Gaye's intimacies. Unfortunately, this bedroom is cut off from the rest of the world, and "Midnight Love" lacks the tense tug-of-war between Gaye's erotic, religious and social concerns that fueled his best albums: the 1971 "What's Going on" and the 1981 "In Our Lifetime."

Prince has made kinky, graphic sex the main subject of his last three albums. At the same time, he has evolved into one of the most innovative talents in pop music. His use of synthesizer dance rhythms and textures is light years ahead of British new-wave bands like the Human League or Soft Cell. His "1999" album, 68 minutes of music spread over two specially priced discs, puts Prince firmly at the forefront of the burgeoning synth-funk movement. His newest single, "1999," is a model of synthesized music: the catchy melody and steady dance groove never get lost amid the exploding electronic percussion and the sliding sheets of sound. Moreover, Prince's big, instinctive voice and subversive guitar fills keep the element of unpredictability alive among the computer programs.

The double album's first side is excellent with "1999" followed by "Little Red Corvette" (a big beat rocker with cheesy organ, fuzzy guitar and contagious falsetto improvising) and "Delirious" (a synthesizer-rockabilly tune that swoons with romance). The rest of the album suffers by comparison. Though the music is always fascinating and full of discoveries, the lyrics simply glorify Prince's sexual swagger. His details are quite specific, but they've lost their force after eight album sides of elaboration. It's time for Prince to move on.

Where he could go lyrically is suggested by his previous album, "Controversy," which brilliantly counterpoised sexual freedom against political repression and violence. "1999," "Free," "Let's Pretend We're Married" and "Lady Cab Driver" from the new album make fleeting references to this conflict, but never make the choices in the street as clear as the options in the bed. Where Prince could go musically is suggested by "How Come You Don't Call Me Anymore?" which is the flip side of the "1999" single but is not on the album. With its old-fashioned soul ballad vocal and its ringing piano figure, it's better than anything on the album and one of the best soul performances of the year. The vocal makes astonishing leaps and dives and firmly links sex, romance, pain and joy together. These are the challenges Gaye and Prince should pursue in the future.