Rehnquist, taken to the hospital Tuesday night, "was admitted for observation and tests," Supreme Court spokeswoman Kathy Arberg said.
Rehnquist, 80 and ill with thyroid cancer, has been tracked in recent days by journalists awaiting word that he may retire. Justice Sandra Day O'Connor, 75, announced this month that she would be leaving the nine-member court, which is final arbiter of whether U.S. laws are acceptable under the Constitution.
He been coming to the court daily until Wednesday. Court officials, questioned about his absence, revealed that he was in Arlington Hospital outside Washington.
Confirmation of the hospitalization followed unusual comings and goings at Rehnquist's suburban Virginia town home. A court police officer made several trips to the house, leaving each time with various personal items. The officer first carried out Rehnquist's distinctive cane and a shirt. Later, he brought out shoes and pants.
It was the second time in less than four months that Rehnquist was taken to the hospital by ambulance. In March, he was taken with breathing problems.
There has been much speculation that Rehnquist might retire soon and create a second vacancy on the bench.
On Tuesday, President Bush conferred withTuesday about candidates to replace Justice Sandra Day O'Conner on the U.S. Supreme Court.
Addressing the retirement speculation, Chief White House Correspondent John Roberts notes that having a second spot to fill on the bench might make it easier for Mr. Bush.
"Having to fill two positions gives the President latitude to expand his horizons,'' says Roberts.
"Simply replacing Sandra Day O'Connor presents a host of complications for the President - does he name a conservative and provoke a fight with Democrats, or does he name a moderate and provoke a fight with his own party's right wing? Either choice will be problematic. It's no secret that President Bush would love to put his long time friend Alberto Gonzales on the court - but conservatives have branded him 'unacceptable'."
Once scenario, Roberts suggests, might be the president nominates Alberto Gonzales to fill O'Connor's position. Liberals are satisfied, but conservatives angry.
The President then chooses a known conservative like J. Michael Luttig to fill the vacancy left by Rehnquist - but only as an associate justice.
Because he nominates Antonin Scalia for chief justice. Conservatives are happy again.
Of course, that means three confirmation battles before the first Monday in October, Roberts notes. Is that a political gamble President Bush would want to risk?