Sen. Daniel Inouye is under observation at a Washington-area hospital so that doctors can monitor his oxygen intake.
The Hawaii Democrat, 88, is the longest serving Senator, now completing his 50th year in the chamber.
He said in a statement that he is, quote, "for the most part, okay."
Inouye said that he is working with his doctors to regulate his oxygen intake and needs to be in the hospital for his own safety and for observation.
Inouye, a decorated World War II veteran, has long been a dominant force in Hawaii politics. He chairs the powerful Appropriations Committee and is the Senate's president pro tempore, third in the presidential line of succession. He served on the Senate's Watergate committee and the House-Senate Iran-Contra panel in the late 1980s.