Yesterday, for the third straight day, there was an emptiness at the center of the Supreme Court.

In the middle of the bench, Chief Justice William H. Rehnquist's chair stood empty, as Rehnquist remained absent from oral argument while recovering from surgery on his damaged right knee.

It is believed to be the first time the chief has been out since a three-day stretch in October 1995, when he was recovering from back surgery, court officials said.

He is still voting on each case, based on the briefs and the argument transcript. But things in the courtroom aren't the same.

From the high-pitched whistle that heralds the arrival of the nine justices to the final gavel that signals their exit, court ritual bears the stamp of the jurist who has been in charge for the last 16 years.

Brevity and punctuality are the hallmarks of Rehnquist's style. A lawyer had better be ready to start the instant the chief calls his name, followed by the cue: "We'll hear from you." Lawyers may make no flowery introductions of themselves and their clients, as they sometimes do in lower courts; a simple "Mr. Chief Justice, and may it please the court" will do.

Without Rehnquist, though, all the actors seem to be ad-libbing.

As the most senior associate justice, John Paul Stevens presides.

Under his mild-mannered stewardship, things move along normally. But yesterday he forgot to say, "The case is submitted" at the conclusion of the first of two oral arguments, leaving audience members and lawyers to figure out for themselves that they were free to stand and stretch during the changeover. Calling a lawyer to the podium, Stevens made a most un-Rehnquistian offer: "Mr. Robbins, whenever you're prepared you may proceed."

And, of course, without Rehnquist's staunchly conservative views reflected in the questions from the bench, it is more perilous than usual to guess how the court will rule based on what the justices say.

Certainly, the second case argued yesterday, Chavez v. Martinez, No. 01-1444, which deals with the scope of an individual's right not to answer police questions, would have provoked inquiries from Rehnquist, given the implications for law enforcement.

In 1997, farmworker Oliverio Martinez was shot and seriously wounded in a struggle with Oxnard, Calif., police. As he lay racked with pain on a hospital gurney, police Sgt. Ben Chavez, investigating the police shooting, questioned him for 45 minutes despite Martinez's insistence that he didn't want to talk.

Martinez later sued, alleging among other things that Chavez's questioning, which did not include reading Martinez his rights, amounted to an attempt at coerced self-incrimination -- even though Martinez was not eventually charged with a crime.

Lower courts upheld Martinez's claim, but Chavez has appealed to the Supreme Court, arguing, in part, that there was no violation of Martinez's constitutional rights since they apply only if statements are used against a suspect at trial.

Civil libertarians say a ruling for Chavez could permit police to abuse people they may not want to charge with a crime, but do want to extract information from.

The Bush administration, however, supports Chavez, suggesting that a ruling against him could tie officers' hands when they may need information quickly.

Justice Antonin Scalia wondered about the case's implications for fighting terrorism. "Let's assume you think someone is going to blow up the World Trade Center. Could the police beat him with a rubber hose?" Scalia asked.

"I understand the terrorism situation is a difficult one, but that's not our case," Martinez's attorney, R. Samuel Paz, replied.

Stevens demanded to know from Deputy Solicitor General Paul Clement whether there is "any protection against the police just grabbing someone off the street and beating him up to get information when they have no intention of using it at trial."

Clement responded that general prohibitions against police conduct that "shocks the conscience," rooted in the constitutional guarantee of due process of law, are already in place, so an additional one based on the Fifth Amendment would be excessive.

A decision in the case is expected by the end of June.