Johnson, a centrist elected to the Senate 10 years ago, became disoriented during a midday conference call with reporters. He was taken to George Washington University Hospital, where he was evaluated for a "possible stroke," a statement from his office said.
The hospital would provide no details on his condition or the reason for the surgery.
A spokeswoman for the senator said Wednesday afternoon that doctors had found no evidence that Johnson, who turns 60 in two weeks, had suffered either a stroke or a heart attack.
Senate Democratic leader Harry Reid of Nevada issued a statement wishing Johnson "a full recovery."
If Johnson is not able to serve when Congress convenes in January, the Senate -- which Democrats narrowly captured in November's election, 51 to 49 -- could revert to GOP control.
Under South Dakota law, Gov. Michael Rounds, a Republican, would appoint a replacement, presumably from his own party -- giving each party 50 seats. Because an evenly divided Senate would give the tiebreaking vote to Vice President Dick Cheney, Republicans would control the chamber.
Johnson's term is up in 2008.