CNN

WASHINGTON (CNN) – Sen. Robert Byrd, the Senate's longest-serving member, was admitted to Washington's Walter Reed Army Medical Center for observation Tuesday after a fall at his home Monday night, his spokesman said.

The 90-year-old West Virginia Democrat complained about pain after going to work Tuesday, and doctors are looking for signs of broken bones or other injuries, Byrd spokesman Jesse Jacobs said.

Byrd cast a vote on the Senate floor and was back in his office working when aides recommended he see the Capitol physician, Jacobs said.

He is the Senate's longest-serving member, first elected in 1958. Though he walks with the aid of two canes, he still speaks vigorously on the Senate floor.

As the senior senator in the majority party, he also serves as the chamber's president pro tempore - third in line in presidential succession, behind Vice President Dick Cheney and House Speaker Nancy Pelosi.

His hospitalization came the same day that another of the Senate's senior members, Virginia Republican John Warner, returned to a Fairfax, Virginia hospital for observation after a recurrence of a heart condition first diagnosed in October. Warner's office said the 81-year-old senator's condition, known as atrial fibrillation, is common and doctors would monitor his heart and adjust his medication during his stay.

–CNN Congressional Producer Ted Barrett