It's unclear what ailed the 80-year-old Democrat from Vermont.
Spokesman David Carle provided an update around 9 p.m. saying Leahy had returned home.
"This evening, Senator Leahy was in his Capitol office and was not feeling well. He was examined in the Capitol by the attending physician," Carle said earlier in the day.
"Out of an abundance of caution, the attending physician recommended that he be taken to a local hospital for observation, where he is now, and where he is being evaluated."
"The president pro tempore has historically presided over Senate impeachment trials of non-presidents. When presiding over an impeachment trial, the president pro tempore takes an additional special oath to do impartial justice according to Constitution and the laws. It is an oath I take extraordinarily seriously," he said.
Leahy has served in the U.S. Senate since 1975.