While there was no immediate word on the condition of the 59-year-old senator, the source, who is familiar with Johnson's situation, said surgeons sought to remedy an unspecified medical problem.
Just hours after Johnson was admitted to George Washington University Hospital, a spokeswoman for him said the senator had not suffered a stroke. She provided no other details.
The actions prompted concerns about his fellow Democrats' razor-thin majority in the incoming Senate.
As a result of last month's elections, Democrats would control the Senate 51-49 when the 110th Congress convenes on January 4.
The consequences of a permanent disability to Senator Johnson has the Democrats worried.
If the senator had to be replaced, South Dakota law says Republican Gov. Michael Rounds would name someone to finish the final two years of his six-year term. An election for a successor would be held in November 2008.
If Rounds named a Republican, that would put the Senate at 50-50, with Vice President Dick Cheney breaking any tie and putting Republicans in charge.
A picture of my cave
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