LOS ANGELES -- Life in the fast lane is finally catching up to rock music's dinosaurs, those aging '60s and '70s performers cashing in on nostalgia tours and the devotion of gray-haired, potbellied fans.


These days, it's rock me gently.


Medical problems have recently sidelined such venerable performers as John Mellencamp, Jerry Garcia of the Grateful Dead, Glenn Frey of the Eagles and David Crosby of Crosby, Stills and Nash.


The culprits: aches and pains from normal aging, the rigors of the road and, in many cases, the cumulative effect of drug and alcohol abuse.


"In their 20s these guys thought they were immortal," Grateful Dead spokesman Dennis McNally said from a backstage telephone in Philadelphia.


"The facts are an entire generation of rockers are pushing 50 and if you aren't living a healthy lifestyle, you'll pay the price," McNally said.


Some of the illnesses are simple bugs. The Who's 50-year-old Roger Daltrey -- who once sang he hoped he'd die before he grew old -- was ordered to take a few days off because of respiratory problems.


Other ailments, however, are more serious.


Frey, 45, has suffered for years from diverticulitis, a painful inflammation of the colon. He underwent colon surgery Thursday and was listed in fair condition.


The surgery forced postponement of the Eagles' concert tour and led to a slew of jokes like this revised song list: "Welcome to the HMO California," "Life in the Gas Pains" and "I Get a Diseaseful, Queasy Feeling."


Doctors say the illness that sidelined Frey, a fitness buff, generally has nothing to do with lifestyle.


That's not the case with some of his colleagues, who are facing the consequences of having lived too hard for too long.


Garcia, 52, is a prime example. His long, strange trip took an extended rest stop in 1991 when he fell ill with exhaustion. Since then, he has worked at slimming down, has successfully stopped smoking and hired a personal fitness trainer.


"He pushed it as far as anybody did," McNally said. "It was a meltdown. Too many cigarettes, too much junk food and too little exercise."


The same goes for Mellencamp, 42, who was forced to cancel his 1994 tour following a mild heart attack and diagnosis of a clogged artery in his heart.


"The moral of my story is that 80 cigarettes a day and a cholesterol level of 300 is like a loaded gun," Mellencamp said.


Crosby, 53, has paid the price for decades of drug abuse that included heroin and cocaine addictions.


"David has been very candid through much of his adult life about the difficulties he's had medically as a result of the use of substances over the years," said his publicist, Elliot Mintz.


"Rock 'n' Roll is not always the easiest experience, to be trekking across the country when you are over 50," he said.


There are, of course, exceptions. The Rolling Stones are still going strong after four decades. Their Voodoo Lounge tour, with $103.5 million in receipts, was the highest grossing tour ever. Much of that had to do with the $50 ticket prices; there were no senior discounts.


Other aging but mostly healthy rockers on tour this year included Pink Floyd, Eric Clapton and Elton John. Thanks to them and their legions of older fans, "1994 will be a banner year for the concert business, some $1.5 billion," said Gary Bongiovanni of the weekly concert trade magazine Pollstar.


"For quite some time now we've been driven by acts from the '60s," he said. "There aren't many young headliners in these stadiums."


And just think how much brighter the financial picture could be if these stars spent more time on stage than in surgery. The Eagles tour had grossed $80 million and was on a record-breaking pace, Bongiovanni said.


"The Eagles might also have been over $100 million if Frey hadn't got ill," he said. "They may have had the record."