Sen. Joseph R. Biden Jr. (D-Del.) was listed in serious condition yesterday following "urgent" surgery to repair a major artery that had burst at the base of his brain, a surgeon and a hospital official said.

Biden, 45, chairman of the Senate Judiciary Committee, was "awake and alert and progressing satisfactorily," said Peter Esker, a spokesman at Walter Reed Army Medical Center. "He is as comfortable as possible and conversing with members of his family. His wife was at his bedside. He was talking with his wife," but remained "seriously ill."

Biden underwent surgery Friday for several hours, his spokesman, Peter Smith, said. The senator was moved to Walter Reed by ambulance Thursday from St. Francis Hospital in Wilmington, Del., following a preliminary diagnosis of an aneurysm and weeks of persistent pain originally believed to be a pinched nerve in his neck.

The pain stopped Biden from participating in the Feb. 3 Senate vote confirming Judge Anthony M. Kennedy to the Supreme Court.

Walter Reed officials declined to discuss details of the operation, but a neurosurgeon, one of several consulted by Biden's surgeons, said the senator's pain was related to the aneurysm -- a swollen artery that hemorrhaged at the base of his brain, not in his neck as earlier reported by the hospital.

The neurosurgeon, who requested anonymity, said: "The purpose of the operation is to eliminate the aneurysm so that . . . the possibility of bleeding is eliminated.

"An aneurysm is a point of weakness on an artery on the surface of the brain. It is this point of weakness that ballooned out like a sack with a thin wall, which eventually can lead to its rupture and bleeding," he said.

On Sept. 23, Biden dropped out of the race for the Democratic presidential nomination following revelations of plagiarism in law school.