The 96-year-old Republican was admitted Saturday night to Washington's Walter Reed Army Medical Center after complaining of fatigue.
He returned to Capitol Hill in time to open the Senate session at 12 noon.
Thurmond, the oldest senator ever, was hospitalized at the same facility last month, two days after a brief stay in a hospital in his home state, where he had collapsed at a University of South Carolina reception.
Thurmond had surgery for an enlarged prostate during that August stay at Walter Reed, a procedure originally scheduled for November.
Press secretary John DeCrosta said the senator has made an estimated five previously unreported trips to the medical center on weekends in the last three months for a variety of complaints. He said none required an overnight stay or caused him to miss any work or Senate votes.
Physicians wanted to find out why he has been fatigued, DeCrosta said in a statement. "Doctors will also evaluate the various medications Thurmond is taking and may decide that changing prescriptions and dosages will assist him."
Thurmond, in a Charleston, S.C., newspaper interview, recently described his health as "generally all right."
"I'm not planning to die soon," he told The Post and Courier. "I expect to serve my term out."
A member of the Senate since 1954, Thurmond's current term ends in 2002.