The senior senator was described as waving as he was being taken away by wheelchair, Senate officials said. He was later taken to Walter Reed Army Medical Center by ambulance.
Shortly after 10:30 a.m. EDT, Thurmond slumped over at his desk, then was helped to a lying position on the floor by several people. Senate GOP Leader Trent Lott told reporters Thurmond was able to stand before he was taken to the ambulance, an oxygen mask on his face.
Sen. Wayne Allard, R-Colo., was standing next to Thurmond when the senior senator tried to stand with the help of an aide and collapsed back into his chair, nearly unconscious, said Allard spokesman Sean Conway.
Allard and staffers then helped lay Thurmond on the floor.
Thurmond was in the chamber as the Senate was debating a defense bill when the incident occurred. When he collapsed, most Republicans were in a closed-door senators' meeting a few paces down the hall from the Senate chamber.
Sen. Bill Frist, R-Tenn., a heart surgeon, immediately left the meeting and ran to Thurmond's aid. When he got there, Thurmond was lying in the aisle near his desk at the front of the chamber with three people surrounding him.
"Dr. Frist checked his response and the best way to describe his condition was that he was woozy," said Sen. Wayne Allard, R-Colo.
After Thurmond's legs were raised, "he started getting less woozy," Allard said.
The Senate recessed for 20 minutes after Thurmond fainted, and Majority Leader Tom Daschle, D-S.D., ordered the chamber cleared.
Capitol police then clamped an extraordinary ring of security around the Senate chamber, the corridors outside and even the parking lot outside the Capitol.
"Are they really making all this fuss for me?" Thurmond said, according to Frist spokeswoman Margaret Camp.
Thurmond, born in December 1902, is legendary for both his political and physical endurance. He was first elected to the Senate in 1954. A one-time Democratic segregationist, he holds the record for a solo Senate filibuster. He has gradually scaled back his duties in recent years as his health declined.
In 1996, at the age of 93, he became the oldest person ever to serve in Congress, and the following year, became the longest-serving member.
When Republicans controlled the Senate from 1981-87, Thurmond was at the peak of his power. He was both chairman of the Senate Judiciary Committee, and, as the longest-serving member of the majority party, also president pro tem, a chiefly ceremonial job that made him third in line to succeed the president.
When the Republicans regained a majority in 1995, Thurmond again was president pro tem and became chairman of the Senate Armed Services Committee, where he pushed for increased military spending.
He steppe aside as chairman two years later as age began to take its toll. When Democrats regained control of the Senate earlier this year, he also turned over the title of president pro tem to Sen. Robert C. Byrd, D-W.Va.
While Thurmond has been to the hospital several times - including a February stay for fatigue - this was the first time health problems affected him while in the Senate chamber.
How long Thurmond will remain in the hospital was not immediately known, said his spokeswoman, Genevieve Erny.
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