U.S. Sen. Robert C. Byrd was readmitted to the hospital for tests following a reaction to antibiotics he was taking to treat a urinary tract infection.
The 90-year-old West Virginia Democrat entered the hospital Wednesday. Byrd's office didn't identify the hospital.
Byrd spokesman Jesse Jacobs said doctors want to run tests to determine which medicine is best to treat him.
Byrd was hospitalized last week at Walter Reed Army Medical Center in Washington, D.C., after suffering a fall at his Virginia home. He was released Monday.
Jacobs said Byrd's latest stay is expected to be brief.
Byrd is the longest-serving senator in U.S. history. As president pro tempore, he's third in line to the presidency. He's also chairman of the Senate Appropriations Committee.