A Kennedy representative, who spoke on condition of anonymity because the announcement was not official, said the Massachusetts senator was in good spirits and that doctors wanted him to get some rest.
Doctors on Tuesday had blamed fatigue for the seizure. Kennedy, who has been under treatment for a brain tumor since last spring, suffered the seizure during an inaugural luncheon after attending the swearing-in of President Barack Obama.
He talked with family and friends and felt well while he was at the hospital, according to a statement released by his office on Tuesday.
The 76-year-old Democrat was diagnosed last May with a particularly aggressive type of brain tumor and has been treated with surgery, radiation and chemotherapy.
Kennedy's seizure at the inaugural lunch was witnessed by fellow senators. Longtime friend Sen. Chris Dodd, D-Conn., quoted Kennedy as saying, "I'll be OK, I'll see you later" as he was put into an ambulance on Tuesday.
Kennedy had appeared in good health and spirits a few hours earlier when he stepped out of the Capitol and onto the inauguration platform where Obama took the oath of office. At the luncheon, Obama told lawmakers his prayers were with the senator and his family.
Kennedy has pledged to forge a breakthrough on health care reform working with Obama and Democrats who control Congress.
Kennedy has been a leading champion of health care reform for decades.
A doctor not connected with the senator's care, Dr. Matthew Ewend, neurosurgery chief at the University of North Carolina, said it's not unusual for patients recovering from brain tumors to suffer seizures.
