"If IT'S successful, it's overdone already," says pop prognosicator Kal Rudman and, as usual, he's right. Rudman, whose tip sheet, Friday MorningQuarterback, tracks record industry trendsm is talking about Medleymania, the latest radio and dance club gimmick. Capitalizing on nostalgia and disco-inspired dance fever, good medleys become instant pop hits. In the last few months:

* A Dutch-produced "Stars on 45" medley of Beatles tunes spawnednot only a No. 1 single and a top-10 platinum album, but renewed acceptance of a marketing ploy dating back to the '40s. The original scored mightily around the world, going top 10 in a dozen countries. Brand-new "Stars on 45" medleys of Abba and '70s pop hits are already on the charts.

* The Royal Philharmonic's "Hooked on Classics," snatches of popular melodies by Mozart and Tchaikovsky set to a booming disco beat, has been such a huge hit that it helped subsidize the Philharmonic's expensive London concert series.

* And currently on the international market are medleys of the Supremes, Credence Clearwater Revival, Beach Boys, Bee Gees, Elvis Presley, the '60s, Hollies, Rod Stewart, Sam Cooke, Four Seasons, '76 Punk and Harry Belafonte.

"How many times can you do the same trick?" Rudman asks. As long as the public buys and the radio plays. Medleys provide an inexpensive means of satisfying an older radio audience's thirst for nostalgia. Much of that audience has moved from the concert hall and record store to home entertainment centers or top-40 lounges where cover bands, which do other people's material, pump out radio hits.

Radio rules the roost, and with the medium moving to a more mature base, medleys are a handy, temporary tool for transition. "I can't believe it's a long-range deal," says Kent Burkhart of the Abrams/Burkhart radio programming syndicate. "But the demographic audience, particularly the adult side, approves of bringing back oldies. It's part and parcel of the continuing 'oldie wave.' "

Recorded medleys have been around since Decca's "Songs of Our Times" series in the '30s and '40s. The records consisted of medleys of the popular songs of each year, three songs to each 78 disc. In 1956, there was a big hit built around "Moonglow" and "Theme From 'Picnic,' " in which each song was played separately and then together, since they were based on the same chord changes. And as recently as three years ago, Cafe Creme put out a medley of 20 Beatles themes. But nothing clicked until "Stars on 45."

Right now, medleys come in two distinct formats, covers and originals. Producer Jaap Eggermont's "Stars on 45" medleys are eerily accurate despite the broad range of artists covered by the session singers who constitute the Stars group. Like most successful Europop producers, Eggermont (one-time drummer for GoldenEarring and Holland's top producer) has an ear for the little tricks of production and recording that make great pop records distinctive. The first "Stars on 45" actually was devised as a countermove to a bootleg dance-club compilation record; it featured perfect mimicry and perfect mixing, an unbeatable dance-floor combination.

The sequel "More Stars" runs through 26 song snatches (ranging from seven seconds to 1:05) in just over 12 minutes. Another 30 song snatches fill out the album ("Stars on 45 II"), prompting the Dutch record company's president to say, "With so many titles per record, it's a bit of a headache, copyright clearance-wise." But with platinum returns, "it's the sort of headache I can force myself to put up with." In storybook fashion, four of the session singers have been invited to New York to perform in a musical-based-on-the-records that may be choreographed by Tom ("Hair," "Jesus Christ Superstar") O'Horgan. Shades of "Beatlemania!"

With the success of cover medleys, some record companies decided they could achieve the same effect by dicing and splicing original tapes in their own studios. Following on the heels of imitation "California Gold" and the imported "Beach Boy Gold," Capitol released its own "Beach Boys Medley." The medley made No. 12 on the singles chart, the highest any Brian Wilson-penned material had gone since 1967. There's an argument to be made that the Beach Boys' live shows have been nothing but a long medley since 1965, so maybe it's a case of the format finally catching up to the material.

Live medleys, have always been part of show business, serving two purposes. For any artist with an enduring career, the thought of having to perform early successes in their entirety is enough to curdle the blood; medleys provide just enough nostalgic fragrance without making you wait for the bloom. And many artists use the format to pay tribute to an era, a genre, a particular performer, or a theme -- witness Bruce Springsteen or Bob Seger.

Airplay and club play provide another incentive. In Europe, dance clubs are still big business, so it's not surprising that the most successful medleys there involve covers segued over strong dance tempos, complete with syndrums, incessant hand claps and throbbing bass lines.

In an effort to garner broader airplay, newer medley records are dispensing with the disco flavor, while major record companies are dispensing with impersonators and using original masters of artists who are in many cases no longer with those labels. Motown's "Diana Ross and the Supremes Medley," originally released last year to resounding apathy, was rereleased after "Stars on 45" and became a moderate hit. Fantasy recently released a mediocre Credence medley; it helped that the band maintained a basic instrumentation, key and rhythm on most of its big hits.

The covers won't go away entirely, though. Tight Fit's "Back to the '60s" and Power's "Play It Again, Sam" will be followed by a 14-song Hollies medley called "Holly Daze"; a Rod Stewart medley by sound-alike Peter Koelewyn called "Rock Heroes"; Lobo's "Caribbean Disco" based on old Belafonte hits; and "Star Trax" from Bee Gees imitators. Successful English and European medleys include "A Tribute to the Punks of 1976," "Hot Licks" and "Dance On," the latter two tributes to The Shadows, a great British band hardly known in America.

The material won't always be tried and familiar, if Polygram has any luck with its unique variation on the medley record, four songs from Martin Briley's debut album mixed together as the B-side to his single, "Slipping Away." According to Polygram executive vice president Bob Sherwood (who has since left the company), the 5 1/2-minute medley may "spark something subtle but significant for the industry. We're hoping it will counteract a negative syndrome that has surfaced recently in which singles buyers avoid albums, having been burned too often by LPs that fail to live up to their single's promise."

"Hooked on Classics" may also be a little different than most medleys, particularly since its composers predate electricity, rock 'n' roll aside. The Royal Philharmonic has been cashing in on rock's appeal for a long time; its series of classical rock albums have accounted for 2 1/2 million records sold, as well as many sellout concerts for a new breed of fans.

"We enjoyed making the record very much," the orchestra's personnel manager told one publication. "We could hear the disco beat banging away in our headphones as we played, and it made us bounce more than usual."

All the way to the bank, it seems, and in the music business, that's less the key than the mold. Best of all, the medley mentality may inspire people to go back to the originals.

Or join the source. Literally. Londoner Adrian Baker had a hit with "Beach Boy Gold," which prompted the band to invite him to America; he ended up touring with them for two months, replacing Carl Wilson when he opted for a solo career. Baker's back in England now . . . writing new material in the old style . . . for the Beach Boys.

Art imitates art.