But in his Monday appearance on the "Late Show with David Letterman," Stern let his loathing rip. A long-time Letterman loyalist, Stern lavished praised on the "Late Show" host and went on the attack against Leno, the former host of the "The Tonight Show."
"I never liked Jay. I can't stand Jay," Stern said. "I've never seen anybody who behaves like a robot like this guy. I watched his final show, saying goodbye to the 'Tonight Show,' reading it off a teleprompter for crying out loud. Where's the emotion? Where's the humanity?"
Pointing at Letterman, Stern then shouted, "Here's the host that we want to watch."
The audience cheered, the band played and Stern danced a little jig. Letterman, who smiled throughout Stern's tirade, sat back in his chair as if to take in the praise, then said, "Thank you, Howard."
In previous visits to Letterman's show, the host has appeared slightly uncomfortable during Stern's attacks on Leno. Not last night. Stern was only on the set a couple minutes before he addressed the recent shuffle in late-night television.
"This Conan O'Brien took over the Leno show. You heard about this?" Stern asked to laughter. "We got to beat this Conan. For God's sake, how are you feeling? Jay left late night television and now you've got this new guy to compete with?"
Rather than answer, Letterman changed the subject. But Stern brought it back around.
"I've got to say something about loyalty. Dave put me on national television many, many years ago before I was known, and I have stuck with Dave," said Stern, whose first "Late Show" appearance in 1984 helped launch the original shock jock and his radio show into the national spotlight.
Even during friendlier times, when Stern appeared on Leno's show, the two never saw completely eye-to-eye. Stern told Letterman on Monday night about one of his appearance on "The Tonight Show."
"I bring on two lesbians -- I'm Howard Stern, I bring out two lesbians -- he's shocked," Stern recalled. "He walks off his own show. I knew it wouldn't last."
Stern vs. Leno -- Who's More Relevant?
Stern vented his ire on Leno earlier this year after the late-night host commented to the New York Times about Stern's diminishing relevance after the shock jock's move to satellite radio three years ago.
In December, Leno was quoted as saying: "Then he (Stern) went to Sirius, and obviously it was a lot of money. But I don't hear him quoted anymore. People don't say: 'Hey, did you hear what Howard said today?' And I'm sure he's still doing what he always does. But to me that seemed a mistake."
On his first day on the air in 2009, Stern singled out Leno: "Jay Leno, with all of his horses**t has 5 million viewers. We on the other hand, because we are listened to by 60 percent of Sirius and XM, we probably have ... 5 to 6 million listeners. ... So if Jay Leno is relevant, I'm surely as relevant as he is."
Over the years, Stern has criticized Leno for ripping off bits from his radio show, like on-the-street interviews, and Stern blasted him after Leno hired Stern's longtime sidekick "Stuttering John" Melendez as an announcer in 2004.
"How much more material are you going to take from me?" Stern said on his radio show. "You make $14 million a year, and Letterman makes $31 million. Why don't you ask NBC for the 31 and pay me the rest since I'm writing material for you now?"
Stern declined Leno's invitation to discuss the matter on "The Tonight Show," and with Leno now passing hosting duties to O'Brien, any chance of the two publicly airing their differences has all but disappeared.