Rehnquist, 80, was taken to the Virginia Hospital Center on Tuesday for tests and observation.
After 33 years on the court and a battle with thyroid cancer, the ailing justice's plans for retirement have been the source of much speculation.
Rehnquist was first hospitalized in October, where he underwent a tracheotomy and has since received chemotherapy and radiation treatment. He has said very little about his prognosis or future plans on the bench.
