Sen. Robert C. Byrd, the longest-serving senator in U.S. history, is back in a hospital for tests after a reaction to antibiotics.

Spokesman Jesse Jacobs says doctors want to run tests to determine which medicine is best to treat Byrd for his reaction to drugs taken for a urinary tract infection.

Byrd's office is not saying which hospital the 90-year-old Democrat is in. His stay is expected to be brief.

Byrd was hospitalized last week at Walter Reed Army Medical Center in Washington after a fall at home. He was released Monday.

As president pro tempore, Byrd is third in line to the presidency. He also leads the Appropriations Committee.