NBC has aggressively denied recent reports that it plans to replace Jay Leno soon as host of “The Tonight Show,” but the network’s top entertainment executive, Robert Greenblatt, recently stirred up a bit of a feud with the late-night star over jokes about the network.

Three executives close to the situation reported this week that Mr. Greenblatt had taken offense to monologue jokes Mr. Leno made last month in the wake of news stories about NBC’s ratings struggles and how the network had fallen into fifth place in the sweeps month of February — behind the Spanish-language network Univision.

Specifically, on the night of Feb. 28, Mr. Leno referenced the news about NBC’s falling into fifth place by telling a series of jokes:

“For the first time in history NBC is going to finish fifth in the ratings period,” Mr. Leno said. “We are behind the Spanish-language network Univision — or as we call it here in Los Angeles: Cinco de Ratings.” He added a series of rapid-fire jokes about how bad off NBC is including: “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.”

Mr. Greenblatt, who has the main responsibility for the network’s prime-time ratings, fired off an e-mail to Mr. Leno, according to those who were aware of the events, and complained. The executives who know about the conflict all asked not to be identified because of their ongoing dealings with NBC.

After the first e-mail from Mr. Greenblatt, two of the executives who know what transpired reported,there was an exchange of pointed e-mails between Mr. Leno and Mr. Greenblatt.

One of the executives who saw the exchange said that Mr. Leno was taken by surprise by Mr. Greenblatt’s comments and strongly defended himself, citing the fact that late-night stars poking fun “at their masters,” as the executive said, is in the long tradition of late-night comedy. Every late-night star, going back at least as far as Johnny Carson, has taken shots at network fortunes within the nightly monologue.

A representative for Mr. Leno said the comedian would not comment on the situation. Mr. Greenblatt was in meetings and was not reachable Friday night, a spokesman said.

This moment of conflict between Mr. Leno and NBC’s management preceded recent reports — denied categorically by NBC executives — that the network was preparing to make a change, moving out Mr. Leno and bringing in Jimmy Fallon, the star of its 12:35 program “Late Night.”

The rationale cited for the change was concern that ABC’s late-night star, Jimmy Kimmel, who was recently moved to go head to head with Mr. Leno, was staking a claim to the younger viewers that are of most economic value in late-night television.

So far, Mr. Leno has continued to win consistently in almost every rating category, including among viewers between the ages of 18 and 49, the chief target for late-night advertisers. Mr. Kimmel won his first week against Mr. Leno in that category, but Mr. Leno has won every week since.

NBC has not provided much help. The network’s ratings in prime time have hit record lows in the past two months. NBC now regularly finishes first in the ratings in only three places: the evening newscasts, “Saturday Night Live” and Mr. Leno’s “Tonight” show.