Jim Watson / AFP / Getty Images Senator Daniel Inouye, D-Hawaii, pictured here in 2009, was hospitalized on Thursday. "For the most part, I am OK," he said.

Daniel Inouye, 88, the most senior senator in the U.S. Senate, has been hospitalized at Walter Reed National Military Medical Center in Bethesda, Md., where he said he is working with his doctors to regulate his oxygen intake.

“For the most part, I'm OK,” Inouye, a Democrat from Hawaii, said in a statement Monday. “Much to my frustration, I have to remain in the hospital for my own safety and to allow the necessary observation. I will be back on the Hill as soon as my doctors allow it.”

Around the Capitol, Inouye has been seen with a portable oxygen supply.

He was hospitalized Thursday, one day before Pearl Harbor Day. On Friday, he honored the day as he does every year, this time through a press release remembering his time as a young Japanese-American teenager in Hawaii. He wrote:

In 1941, the date December 7th was a day that evoked anger, fierce patriotism and dangerous racism. Soon after that day, I suddenly found myself, pursuant to a decision by the government and along with thousands of Japanese Americans declared 4C, enemy aliens. It was a difficult time. I was 17.

Inouye serves as President Pro Tempore of the Senate and therefore is third in line for succession to the presidency.

He has served in the Senate for 49 years, since 1963. He is currently the longest-serving member of the Senate; late Sen. Robert Byrd of West Virginia served for 51 years.

Inouye was hospitalized on Nov. 15 after falling and cutting the back of his head. A statement released by his office spoke to the senator’s apparent dislike of being hospitalized: “The U.S. Army Captain and World War II combat veteran wanted to put a bandage on and come to work but his family insisted he get it checked out.”

NBC's Kelly O'Donnell and Isolde Raftery contributed reporting.

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