Press secretary Jesse L. Jacobs said the West Virginia senator would be in the hospital overnight for observation.
Byrd, who is the longest-serving senator in history, voted during a 5:30 p.m. roll call Monday, then went home. Jacobs said the senator began to feel ill less than an hour later.
He was found to have a fever, and at his doctor's request he was taken to a nearby hospital.
Jacobs added that he didn't know which hospital Byrd was taken to.
Byrd was hospitalized March 5 for tests after a reaction to antibiotics. A week earlier he was hospitalized at Walter Reed Army Medical Center after a fall at home.
Last month, Byrd gave an emotional speech on the Senate floor after hearing the news that Sen. Edward M. Kennedy, D-Mass., had a malignant brain tumor. Kennedy, 76, underwent 3½ hours of risky brain surgery at Duke University Medical Center on Monday, an operation his surgeon pronounced a success.
Byrd has become significantly more frail - and sometimes prone to emotional outbursts - since Erma, his wife of almost 69 years, died two years ago. Earlier this year, rumors swirled that fellow Democrats thought Byrd was too feeble to remain at the helm of the Senate Appropriations Committee. Byrd silenced the whispering campaign with a strong performance at a hearing in April.