Two days after snaring an interview with Hillary Rodham Clinton, David Letterman underwent emergency quintuple bypass surgery today in New York and is likely to be off his late-night show for weeks. Doctors tonight said they expect a complete recovery.

The 52-year-old talk show host had gone for an angiogram on Thursday because he has high cholesterol and a family history of heart disease; his father died of a heart attack in his fifties. The procedure, in which a flexible tube is inserted into an artery, revealed the blockage.

Because tonight's "Late Show With David Letterman" was taped on Thursday, viewers heard Letterman disclose that he was going to have the angiogram several hours after he had already had heart surgery.

His guest on the show was "Who Wants to Be a Millionaire" host Regis Philbin, who had emergency angioplasty at age 59 in 1993. Letterman, an avid runner who trimmed down considerably when he moved from NBC to CBS's late night, has made an ongoing joke of his health since discovering that he had a high cholesterol level a few years back. On tonight's taped show, when Philbin asked him about it, Letterman cracked that it was "borderline--680." The recommended blood cholesterol level is under 200.

Letterman noted that there was a chance they would have to "open Daddy's rib cage . . . and I don't want that."

He didn't get that wish, and the operation was performed--not at Home Depot as Letterman had told viewers, but at New York Presbyterian Hospital by surgeon O. Wayne Isom, who forecast the comic's rapid and full recovery, according to the Associated Press.

CBS finds itself unexpectedly looking into a late-night ratings abyss with the February sweeps race just around the corner. The taping of tonight's show was the first that network suits had heard of Letterman's heart problem.

He could spend up to eight weeks recuperating at home, which would leave CBS looking at either reruns or a guest-hosted "Late Show" during the ratings derby. In his 18-year late-night career, Letterman has never had a guest host on either his NBC or CBS shows. CBS said tonight that it would air reruns for the time being.

On the other hand, if Letterman returns to work sooner--Isom said his patient "has the heart muscle of a 20-year-old"--that could spike the show's ratings during the sweeps.

Tonight CBS said it was "thrilled that David's surgery went well" and that while "we are concerned first and foremost about his health . . . we can't wait until he's back behind his desk where he belongs."

The timing couldn't have been worse for CBS, coming as it did two days after Letterman's well-publicized interview with first-lady-in-absentia Hillary Rodham Clinton. That catch yielded 11 million viewers--the largest "Late Show" audience since Letterman's first week on the network, when he pounded NBC's "Tonight Show With Jay Leno."

After several seasons playing a distant second to Leno, Letterman's show finally had been resuscitated and is enjoying its best competitive position relative to Leno in five years, thanks in part to CBS's improved ratings in the 10-11 p.m. hour on several nights.

Leno tonight said that he and his staff wish Letterman a speedy recovery.

Letterman broke into network TV on CBS in 1978 as a guest on Mary Tyler Moore's variety show, "Mary." He made appearances on "The Tonight Show" when Johnny Carson hosted it, and guest-hosted many times before Leno became Carson's regular substitute. After a brief stint as host of the ill-fated daytime "The David Letterman Show," he debuted NBC's "Late Night With David Letterman" in 1982 in the post-Carson time slot.

In 1993, after Carson retired and NBC decided to give the gig to Leno instead of Letterman, the Gap-Toothed One jumped to CBS.

CAPTION: David Letterman's doctor said the late-night host "has the heart muscle of a 20-year-old."

CAPTION: David Letterman joked Thursday during taping that his cholesterol was "borderline . . . 680."