The court's statement described the procedure as "routine" and noted that it was a "revision" to a previously undisclosed stent procedure Kennedy had last November. Kennedy was released Sunday morning.

That first operation took place at a politically sensitive moment for the court. It was shortly after Chief Justice William H. Rehnquist had died of cancer and was succeeded by John G. Roberts Jr.

And Samuel A. Alito Jr. had just been nominated as President Bush's choice to replace Justice Sandra Day O'Connor, after a turbulent October during which the president's first pick, Harriet Miers, had withdrawn in the face of conservative Republican opposition.

Appointed to the court by President Ronald Reagan in 1988, Kennedy has compiled a mostly conservative voting record but has also voted to uphold Roe v. Wade and in favor of gay rights. Kennedy wrote the court's 2005 opinion striking down the death penalty for offenders younger than 18.

After O'Connor, another moderate conservative, left the court earlier this year, Kennedy emerged as the pivotal swing voter between two four-member blocs of liberals and conservatives.

Yesterday's announcement was the first public acknowledgment that Kennedy has experienced any health problem affecting a vital organ. He had back surgery in 1993 and an eye operation in 1997.

In modern heart surgery, stents are widely used as an alternative to more invasive cardiac bypass surgery, which involves creating new pathways to the heart for blood to flow around blocked arteries.

But in about 10 percent of patients, the stented artery narrows again, a process known as restenosis. In such cases, surgeons must reenter the site and implant a new stent, said Roger S. Blumenthal, a cardiologist who directs the Johns Hopkins Ciccarone Preventive Cardiology Center in Baltimore.

Kennedy's future course of treatment will probably include doses of aspirin and a medicine called clopidogrel to prevent a recurrence, Blumenthal said.

Kennedy showed no outward signs of illness during recent months, attending all oral arguments at the court as usual, keeping up a full schedule of speaking engagements and, in July, traveling to Austria for a seminar on constitutional law, as he does every year.

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