ASHINGTON, March 21 - After an absence of more than five months, Chief Justice William H. Rehnquist returned to the Supreme Court bench on Monday, looking quite fit and participating actively in two hours of arguments.

The only noticeable indication of the thyroid cancer for which the 80-year-old chief justice has been treated since October was the unfamiliar quality of his voice, by turns reedy and husky. He underwent a tracheotomy on Oct. 22 to relieve a breathing obstruction, and the tracheotomy tube is still in place. He is being treated with radiation and chemotherapy.

Two months ago, when the chief justice administered the oath of office to President Bush at his only previous public appearance since he began his treatment, his face was gaunt and he walked with a cane. On Monday, the gauntness was gone and he took his seat on the bench without assistance. He left the bench for a brief period during each of the two hourlong arguments.

Chief Justice Rehnquist appeared fully engaged by each of the cases, asking seven questions during each argument. He has been working in his chambers, as well as from home, since the start of the year, and has voted on cases on the basis of the briefs and argument tapes and transcripts.

Chief Justice Rehnquist, who passed his 33rd anniversary on the Supreme Court in January, is widely presumed to be planning to retire when the current term ends this summer. But he has given no indication that he is in fact contemplating retirement, or what the timing might be if he is.

Doctors not connected with his case said on Monday that his return to the court might indicate that he has a less aggressive form of thyroid cancer than has generally been inferred from his course of treatment. The court has not provided information about the chief justice's specific diagnosis. The most aggressive type, anaplastic thyroid cancer, is usually fatal within months.

"If he does have the most aggressive type, from what I can tell, it certainly would seem that he is doing better than average," said Dr. Steven I. Sherman, chairman of the department of endocrinologic cancer at the M. D. Anderson Cancer Center in Houston. "It's quite possible that his cancer is less aggressive than was generally assumed, in which case his prognosis would be measured in a matter of years, not months."

Dr. Sherman noted that following treatment, many patients with advanced thyroid cancer can return to their normal lives for a time, so it was impossible to conclude that the chief justice has a less virulent form of the disease.

There were these other developments on Monday as the court returned from a two-week recess.

Sept. 11 Appeal

Without comment, the court denied an appeal by Zacarias Moussaoui, the only person charged in an American court with participating in the Sept. 11 attacks. Mr. Moussaoui was challenging a ruling by the federal appeals court in Richmond, Va., nearly a year ago that restored his liability for the death penalty and held that he was not entitled to have access to captured members of Al Qaeda who could provide helpful testimony at his trial.

Federal District Judge Leonie M. Brinkema of Alexandria, Va., had ruled that the government could not impose the death penalty on a defendant who was denied access to favorable witnesses. She said that the government would have to provide a videoconference for depositions from the witnesses, who have told interrogators overseas that Mr. Moussaoui had nothing to do with the plot.

In overturning her decision, the United States Court of Appeals for the Fourth Circuit held that instead of video depositions, to which the government objected, adequate "substitutions" could be devised in the form of written summaries of the witnesses' answers to questions.

In his Supreme Court appeal, Moussaoui v. United States, No. 04-8385, Mr. Moussaoui's lawyers argued that he could not get a fair trial without being able to exercise his right under the Constitution to receive all favorable evidence in the government's possession and to have access to witnesses.

Indicted three years ago, Mr. Moussaoui has not yet gone to trial. Ordinarily, the Supreme Court is reluctant to hear appeals that concern pretrial legal issues.

But his lawyers argued that the ordinary rule should not apply in this case because the Fourth Circuit, in creating "a new national security exception" to a defendant's right to testimony from witnesses, had "devised a novel and flagrant detour around cornerstone constitutional principles."

The Supreme Court's refusal to intervene at this point does not necessarily mean that a trial will take place quickly. Under the Fourth Circuit's order, Judge Brinkema must now devise procedures for summarizing the written testimony of the witnesses, and those procedures may be the subject of further pretrial challenges.

Recess Appointment

The court turned down three cases challenging President Bush's temporary appointment of William H. Pryor Jr. to a seat on the United States Court of Appeals for the 11th Circuit. The president made the "recess appointment" during an 11-day Congressional break for Presidents' Day in February 2004.

Three defendants with cases in the 11th Circuit, which is based in Atlanta, argued that such a brief recess was not the sort contemplated by the Constitution's framers in the recess appointments clause, which gives the president the power to fill vacancies "during the recess of the Senate." Judge Pryor is to serve through the end of the current Congressional session, later this year.

Justice John Paul Stevens wrote a brief opinion in one of the cases, Evans v. Stephens, No. 04-828, to say that the case raised "significant constitutional questions" and to caution against drawing a conclusion that by not taking up the issue, the Supreme Court was implicitly validating the president's action.

"It would be a mistake to assume that our disposition of this petition constitutes a decision on the merits of whether the president has the constitutional authority to fill future Article III vacancies, such as vacancies on this court, with appointments made absent consent of the Senate during short intrasession 'recesses,' " Justice Stevens wrote.

Article III refers to the constitutional provision for federal courts with life-tenured judges.

The two other cases were Miller v. United States, No. 04-38, and Franklin v. United States, No. 04-5858. Other challenges to Judge Pryor's appointment are making their way through the lower courts.