(CNN) -- Sen. Daniel Inouye, who has represented Hawaii in Congress since its statehood, was at a Washington-area military hospital on Monday where his oxygen levels were monitored, he said in a statement.
"For the most part, I am OK. However, I am currently working with my doctors to regulate my oxygen intake," the Democrat said. "Much to my frustration, while undergoing this process, I have to remain in the hospital for my own safety and to allow the necessary observation."
Inouye, 88, who was at Walter Reed National Military Medical Center, said he would be back on Capitol Hill as soon as his "doctors allow it."
Inouye was on the Senate floor last week with an oxygen mask before a ratification vote on a United Nations treaty involving disability rights.
He received hospital treatment last month for a minor head injury following a fall.
Inouye was elected to his ninth consecutive term in 2010. Behind the late Sen. Robert Byrd of West Virginia, he is the second longest serving senator in the chamber's history.
He has been a senator for all but three of the Hawaii's 53 years as a state. Before that, the decorated World War II veteran was its first House member.