Are oldies over?

New York's WCBS-FM, the nation's flagship oldies station, earlier this month killed that format to make way for Jack, the trendy new approach to music radio in which playlists expand from a couple of hundred songs to perhaps 1,000. The idea is to mimic the range and unpredictability of an iPod in shuffle mode.

With a jarring segue from Frank Sinatra's "Summer Wind" to the Beastie Boys' "Fight for Your Right," WCBS, which is owned by Infinity Broadcasting, put a surprise end to 33 years of oldies in the nation's No. 1 market. At the same moment, Infinity's oldies station in Chicago, WJMK, made the same format switch.

Washington's oldies station, WBIG (100.3 FM), which is owned by Clear Channel, took a less dramatic turn in 2002, when it changed its on-air name from Oldies 100 to Big 100 and dropped many tunes from the 1950s and early 1960s, replacing them with a larger selection from the '70s.

But that adjustment may not be enough for radio companies concerned that advertisers have little interest in an audience of aging boomers. WCBS-FM had a strong ninth-place showing in the winter ratings in New York, but its listeners were simply too old for Infinity's tastes. The appeal of Jack -- in March, Washington's Mix 107.3 adopted a lite version of the format, which offers a wide range of pop tunes from the '70s, '80s and '90s -- is twofold: By pushing the oldies concept a decade or two later, it enables stations to reach listeners in their thirties and forties with the tunes of their youth, and by vastly increasing the number of songs played, it lets stations compete against the infinite choice available on iPods and the Internet.

WCBS was one of the last stations in the country that kept doo-wop and other '50s rock hits on its playlist, and it was home to some of New York's legendary Top 40 deejays, including Cousin Bruce Morrow, Harry Harrison and Norm N. Nite. Its morning jock was Micky Dolenz, the former Monkee.

The new format plays songs with no deejays, no news, no weather, no traffic.

If radio's history is any guide, the jocks will be back; radio periodically seeks to lure music lovers by scrubbing its programming of personality, only to find that listeners want more than a pure jukebox.

But the original oldies -- the big hit songs of the '50s and '60s -- may not be back. Just as beautiful music formats featuring the crooners of the big band period and the instrumental elevator music that had large adult audiences during the rock era finally died off over the past decade, oldies stations are starting to move to the staticky ghetto of the AM band (and to the Internet, where there is bandwidth enough for any and all formats).

Some programmers who mold oldies stations believe they have run out of room to modernize the format's sound. Pop music lost its mass foundation about 1974, splintering by race, class and cultural affinity. Rock people and disco people were at loggerheads in the nation's high schools and nightclubs in the late '70s, and no one ever again managed to put together the musical and social coalition that had driven the Top 40 bandwagon for the previous two decades. So programmers who want to update the oldies sound have nowhere to go after the Motown and Beach Boys audience gets too old for advertisers' tastes. Fleetwood Mac turns out to be just about the last band that appeals across the pop culture boundaries.

But Bennett Zier, who as regional vice president of Clear Channel supervises Big 100, says it's too early to write oldies' obituary.

"The format will evolve," he says. "It needs to grow by finding new songs from that period. There isn't a TV commercial out there today without a song from the '60s as its soundtrack. Every movie has songs from that era. This is the greatest music and it won't die."

Zier, who has worked at oldies stations in three cities, including at WCBS-FM, argues that some songs in the format are classics that "just don't burn," the industry term for listener exhaustion with an overplayed tune. "No one gets tired of 'My Girl' or 'Pretty Woman,' " he says. "That music is not going away."

The Jack format tries to blend such favorite songs with surprises from the past 30 years -- tunes that haven't been on the radio for a very long time. By playing a little bit of everything, Jack edges radio back toward the era before stations were rigidly segmented by narrow demographic appeals. The idea is to create the illusion of random choices of songs, crashing Elvis into Bob Marley into Guns N' Roses, or "Dancing Queen" into "My Sharona" into "1999."

Jack may yet prove to be a novelty act, a passing fancy, but oldies are likely to survive only as a fringe format, drawing dwindling audiences to outposts on the Web and on satellite radio, as radio again seeks a way to capture the youngest of listeners.

So long, Sinatra: WCBS used the Beastie Boys to usher in Jack, an eclectic -- and very trendy -- new format.