The Tonight Show host Jay Leno is under fire from NBC executives for making jokes about the network’s recent slide in the ratings.

Reports suggest that Robert Greenblatt, NBC’s Chairman of Entertainment, fired off a series of emails slamming the late night comedian for daring to lampoon the lousy ratings. Leno gave back as good as he got, insiders say, and bristled against the PC, company line criticism.

What offended Greenblatt? On February 28, for instance, Leno kicked off his NBC ratings segment saying, “For the first time in history NBC is going to finish fifth in the ratings period. We are behind the Spanish-language network Univision–or as we call it here in Los Angeles: Cinco de Ratings.”

He went on to joke that, “It’s so bad, ‘The Biggest Loser’ isn’t just a TV show anymore; it’s our new motto.” And: “It’s so bad, NBC called Manti Te’o and asked him to bring in some imaginary viewers.”

Those familiar with the email exchange say that Leno returned a strongly worded reply to NBC execs saying that there is a long tradition of late night comedians taking pot shots at “their masters.”

Recent rumors suggest Leno is under the microscope and about to be replaced by entertainer Jimmy Fallon, host of the network’s Late Night show.

But even as NBC execs slam Leno for picking the ratings scab, Leno’s own ratings have been strong. Unlike the rest of the network, Leno has consistently won his time slot and done strongly with key age demographics.