A prominent federal appeals court judge, best known for joining in an opinion holding unconstitutional the recitation of the Pledge of Allegiance in public schools, was hospitalized today after having difficulty breathing, officials said.

Judge Stephen Reinhardt, 77, a staunchly liberal voice on the 9th Circuit Court of Appeals, was traveling by plane to Washington when he took ill, a spokesman for the court said in an e-mailed statement. Oxygen was administered to Judge Reinhardt and the plane was diverted to Las Vegas, where he was admitted to Desert Springs Hospital, the spokesman, David Madden said.

Judge Reinhardt, who was appointed by President Carter, is scheduled to speak Saturday at a legal conference in Washington sponsored by the American Constitution Society.

“The cause of his breathing difficulties is not known. I am told tests are being run to explore various possibilities,” Mr. Madden wrote. “Judge Reinhardt is an esteemed member of the court and very much in all of our thoughts.”

Judge Reinhardt’s illness was disclosed this morning by the 9th Circuit’s chief judge, Alex Kozinski, who called a brief recess at a trial he is presiding over at Los Angeles, a lawyer present said.

The appeals court’s 2002 opinion about the pledge, which was written by Judge Alfred Goodwin, was overturned by the Supreme Court, on the grounds that the plaintiff in the case, Dr. Michael Newdow, lacked standing. Dr. Newdow, who is a lawyer, refiled a virtually identical case on behalf of others and argued the new case last year before a different 9th Circuit panel, which also included Judge Reinhardt. No ruling has been issued.