Sen. Jesse Helms underwent surgery Thursday to replace a leaking pig valve that was installed in his heart 10 years ago.

The operation began at about 7 a.m. at Inova Fairfax Hospital in Fairfax, Va., said Jimmy Broughton, Helms' chief of staff. It was expected to last at least four hours.

"I talked to him last night and he seemed OK," Broughton said. "He was talking about the nurses and carrying on. They were doing all sorts of things to him, but he was putting up with it."

The 80-year-old Republican could remain in the hospital a week or longer. Broughton said it is hard to predict when he will resume a full schedule.

Helms announced last August that he plans to retire next January after 30 years in the Senate. He said he wanted to spend more time with his family during his twilight years.

He was admitted to the National Naval Medical Center in Bethesda, Md., earlier this week after complaining of feeling weak.

The valve from the heart of a pig was used to replace Helms' mitral valve in 1992. The mitral valve guards the opening between the left upper and lower chambers of the heart.

It's not unusual for the pig valves to wear out and need replacement after a period of years. Doctors continue to use them because, unlike mechanical valves, they don't promote the formation of blood clots.

Helms has had a variety of ailments in recent years.

He had knee-replacement surgery in 1998 and was diagnosed with "peripheral neuropathy," a condition that numbs his feet and impairs his balance, in 2000.

That same year, he was hospitalized with a serious case of pneumonia, and last year he had fluid drained from his lungs after contracting what aides described as "a bad head cold."

Helms is known for his strong opposition of communist regimes and his criticism of foreign aid. He used the chairmanship of the Senate Foreign Relations Committee, which he held from 1995 to 2001, to promote his views.