U.S. Sen. Daniel Inouye has been hospitalized since Thursday after he fainted while in one of his Senate offices, according to his staff.

The Hawaii Democrat, who is 88, had been using an oxygen supplement for the past several months. The senator said in a statement that he is working with his doctors to regulate his oxygen intake. He will likely remain in the hospital for the rest of the week.

After fainting, Inouye was taken to George Washington University Hospital, his staff said. He was moved to Walter Reed National Military Medical Center on Sunday.

This is the second time Inouye has been hospitalized in less than a month. The senator had a minor cut on the back of his head after a fall at his home on Nov. 15 and was taken to the Bethesda Naval Hospital as a precaution. He was treated and released the same day and returned to his home in Maryland.

Inouye, chairman of the Appropriations Committee, is the Senate’s most senior Democrat and serves as Senate president pro tempore, placing him third in line of succession to the presidency. He has served in the Senate since 1963.

"For the most part, I am OK," Inouye said in a statement he dictated to his staff from Walter Reed. "However, I am currently working with my doctors to regulate my oxygen intake. 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. I will be back on the Hill as soon as my doctors allow it. Thank you all for the kind words and continued support."

Inouye, an Army veteran who lost his right arm when he was severely wounded in World War II, has spoken of being a former heavy smoker who was misdiagnosed with lung cancer in the late 1960s. His staff has said that the senator had a sizable portion of his left lung removed in an operation to search for the cancer.

Inouye has been using an oxygen supplement for about nine months per doctor recommendations.

The senator, who frequently walks with a cane, has also been using a wheelchair at the U.S. Capitol to reduce the strain of the marble floors on his knees.

Inouye was re-elected in 2010 to a six-year term that runs through 2016, when he would be 92. He has indicated that he intends to run for re-election.

In an interview two years ago, Inouye said he had instructed his family and staff to closely monitor his health and promised "that if I ever felt that I’m physically not up to it, or mentally deficient, I would quit."

U.S. Sen. Daniel Akaka, who is also 88 and is retiring in January, said in a statement, "I wish my friend, colleague, and brother Dan Inouye a speedy recovery and look forward to his return to the Senate."

Other colleagues also wished Inouye well.

"My thoughts and best wishes to Senator Daniel Inouye for a quick recovery," U.S. Rep. Mazie Hirono, D-Hawaii, who is replacing Akaka in the Senate, said in a statement.

Former City Councilwoman Tulsi Gabbard, who will replace Hirono in the House in January, wished the senator a "full and speedy recovery," adding, "He is in the best of hands at Walter Reed, and I am confident that he will have a quick return to the U.S. Capitol. I look forward to his continued leadership, as we take on the many challenges facing Hawaii and our nation. My thoughts and prayers are with him and his ohana."

U.S. Rep. Colleen Hanabusa, D-Hawaii, said, "I expect to see Senator Inouye back at work shortly. I know he is anxious to get back to serving our state and our nation."

Gov. Neil Abercrombie said in a statement that he is in close touch with Inouye’s office.

"He is alert, feisty and ready to get back to work as soon as possible," the governor said.