Just past 9:30 on a recent morning, Mark and Brian, co-hosts of the morning show on KLOS-FM (95.5), were schmoozing on-air with singer Kenny Rogers about his farm in Georgia.
A little farther up the dial, at KLSX-FM (97.1), the conversation took a less bucolic turn. Howard Stern, who needs no introduction, was ridiculing a newspaper reviewer (her name "is probably Japanese for 'Mickey Mouse,' " Stern suggested) and needling phone-in caller Dick Clark with a mock pitch for a gay version of "American Bandstand."
"I have single-handedly changed the face of radio," Stern told Clark with typical immodesty.
There's at least some truth to Stern's boast, and that's bad news for Mark Thompson and Brian Phelps. Once the rulers of morning drive time in the nation's second-largest radio market, the KLOS duo has watched its ratings gradually slip since Stern brought his brash syndicated show to Los Angeles in July 1991.
The most recent Arbitron ratings showed Mark and Brian's program tied with news outlet KFWB-AM (980) for 11th place among all listeners over the age of 12. (Stern tied with KKHJ-AM  for fourth.) Just 3.6% of the available audience tuned in Mark and Brian, compared to almost 9% during the program's early '90s heyday.
The KLOS morning show's audience share climbs about a point among listeners in the 25- to 54-year-old demographic that most radio advertisers covet, but lately it has been running third or fourth even in that category.
When asked to explain the slide, some experts have a two-word answer.
"Howard Stern," said Dan O'Day, a Los Angeles-based radio programming consultant. "I can remember a few years ago, a program director for a competing station said, 'Mark and Brian are so hot, no one can stop them.' Then Howard came along and took away a sizable chunk of their audience. They do not do the same show, but there is a definite crossover among their audiences."
A KLOS spokesman said that Thompson, 40, and Phelps, 38, did not want to comment for this story. But Carey Curelop, program director at the rock station, admitted that although KLOS is "very pleased" with Mark and Brian's show, their East Coast rival has taken his toll in the ratings.
"Howard has come into the market and taken a portion of the audience," Curelop said. "Quite honestly, that's to be expected."
But the outcome may not have been inevitable. A master of self-promotion as well as of colorful put-downs, Stern has carefully transplanted his funny/obnoxious persona to network talk shows (most notably David Letterman's), cable's E! Entertainment channel and two best-selling books.
Mark and Brian, meanwhile, bombed with an ill-conceived 1991 prime-time show on NBC. They have syndicated their morning program to 20 smaller markets--including San Francisco, Honolulu and Portland, Ore.--but KLOS' most recent billboard promotion for the pair took place last spring.
With Stern siphoning off many listeners, Mark and Brian are struggling in a crowded drive-time pack that includes Pepe Barretto and Lupita Pena on top-rated Spanish station KLVE-FM (107.5), Tha Baka Boyz on hip-hop KPWR-FM (105.9) and Kevin and Bean on alternative rocker KROQ-FM (106.7).
"They're not young, mischievous boys anymore," said Nicole Sandler, who produced Mark and Brian's show from 1991 until 1994, when she and newsman Chuck Moshontz left to become morning co-hosts on KSCA-FM (101.9). "Now [the show] is just kind of old and tired. They're still doing things they were doing six or seven years ago."
The team has hit speed bumps before. When KLOS hired them in 1987, fresh from a morning gig at a Top 40 station in Birmingham, Ala., their listeners flooded the station with angry phone calls complaining that chatter was interrupting the station's hard-rock playlist. "I guess we came on like gangbusters," Phelps told The Times then.
But within months, the act caught on. Fans appreciated the rock-flavored humor (the weight of Heart singer Ann Wilson became a running joke) and special stunts, such as smashing pumpkins that had been carved into caricatures of Jim and Tammy Faye Bakker.
By fall 1991, when NBC unveiled "The Adventures of Mark and Brian" on Sunday nights opposite "60 Minutes," the duo had been the top morning deejays in Los Angeles for a year and a half and were among the most recognizable local celebrities.
The ensuing failure of the NBC show might have been more painful if anyone had seen it. TV critics disdained the pair's incessant mugging and goofy stunts, like trying to sing backup for the Temptations. After a few weeks of poor ratings, the network put the show on "hiatus," the customary TV euphemism for a mercy killing.
Around the same time, their morning show began slowly hemorrhaging listeners. The demands of the weekly TV outing had diverted their attention from radio, Sandler said.
"Sometimes they would get off the air at 10 or 10:30 and they'd be gone, left the station," she said. "And then [preparation for upcoming shows] fell in my lap."