"After testing, we believe the incident was brought on by simple fatigue," Edward Aulisi, Chairman neurosurgery at Washington Hospital Center, said Tuesday night in a statement, adding the Massachusetts Senator is feeling much better.

"Sen. Kennedy is awake, talking with family and friends, and feeling well. He will remain at the Washington Hospital Center overnight for observation, and will be released in the morning," Aulisi said.

Kennedy, 76, was taken in an ambulance to the hospital from the luncheon. It was his fellow Senator from the State, John Kerry, who held his hand tightly throughout the episode.

"He's (Senator Kennedy) in good shape. I'm absolutely confident he's going to be out of here tomorrow. And go on from there," Kerry, Chairman of the Senate Foreign Relations Committee, told reporters outside the hospital.

"His original sickness was diagnosed as a result of a seizure that he had, and he's had a couple of other minor ones that have occurred as a medicine was changed or something took place," he said.

Kerry was with Kennedy when he collapsed. "No, I think that -- look, nobody -- nobody wants to see a friend and a colleague go through a seizure, period," he said.

"I think that he was so upbeat, enthusiastic and excited about the day, I think he didn't want anything to detract from that in any way. And so, you know, I think he's, like all of us, anxious to focus on Barack Obama and focus on the meaning of today," he said.