"He's resting comfortably, and watching the Red Sox game with his family," said Dr. Larry Ronan, his primary care physician. "Over the next couple of days, Senator Kennedy will undergo further evaluation to determine the cause of the seizure, and a course of treatment will be determined at that time."
On Saturday morning, Kennedy felt ill at his home and went to Cape Cod Hospital. After a discussion with his doctors in Boston, the senator was flown by helicopter to Massachusetts General Hospital, where he was soon joined by his wife Victoria, three of his children and his niece, Caroline Kennedy.
Senate Majority Leader Harry Reid said he spoke to Kennedy's wife in the afternoon and was told "his condition is not life-threatening, but serious."
"But the one thing I can say, if there ever was a fighter, anyone who stood for what we as Americans, we as Democrats, stand for, it's Ted Kennedy," Reid said addressing the Nevada Democratic Convention in Reno.
In October, Kennedy had surgery to repair a nearly complete blockage in a major neck artery. The discovery was made during a routine examination of a decades-old back injury.
The hourlong procedure on his left carotid artery - a main supplier of blood to the face and brain - was performed at Massachusetts General. This type of operation is performed on more than 180,000 people a year to prevent a stroke.
The doctor who operated on Kennedy said at the time that surgery is reserved for those with more than 70 percent blockage, and Kennedy had "a very high-grade blockage."
Distinguishing between a seizure and a transient ischemic attack, TIA, often called a mini-stroke, can sometimes be difficult.
Seizures are little electrical storms in the brain. They tend to be brief; an occasional one can happen to anyone even without a prior history of seizures, especially if there has been some prior brain trauma.
A stroke is either ischemic - a clog in a blood vessel - or hemorrhagic, bleeding in the brain. Hemorrhagic ones are very rare. Kennedy had the carotid artery surgery to try to prevent the ischemic type. A stroke kills brain tissue; how much depends on how big it is and how long it lasts. Some people show no lasting effects; others can be partly paralyzed on one side or somewhere in-between.
"Sen. Kennedy was at high risk because he had surgery for an artery in his neck," said Dr. Wendy Wright, The Emory Clinic, Assistant Professor, Departments of Neurology and Nuerosurgery.
But she said there are a lot of things that can cause seizures, such as an infection or medication.
"Certain medications are known to cause seizures. A stroke can cause a seizure, a brain tumor or a head injury, or something in the brain itself," Wright said. "Common symptoms that we know about are falling on the ground, shaking and having confusion."
Kennedy, the second-longest serving member of the Senate and a dominant figure in national Democratic Party politics, was elected in 1962, filling out the term won by his brother, John F. Kennedy.
Kennedy eldest brother, Joseph, was killed in a World War II airplane crash. President John Kennedy was assassinated in 1963 and his brother Robert was assassinated in 1968.
Kennedy is active for his age, maintaining an aggressive schedule on Capitol Hill and across Massachusetts.
He has been vocal in both his opposition to the Iraq war and support for Democratic presidential contender Barack Obama, who is trying to become the first senator elected to the White House since John F. Kennedy.
Kennedy made several campaign appearances for the Illinois senator in February, and most recently another in April.
Always concerned about maintaining his health, Kennedy regularly consults with a battery of Massachusetts General doctors.
Still, he maintains homes in both Boston and Washington and attends not only official events, but numerous others recognizing his family's political history.
Just last week, he and Caroline Kennedy awarded the annual "Profiles in Courage" award commemorating President Kennedy. And Friday, he attended a ribbon cutting at the New Bedford Whaling National Historical Park.
He was preparing to host the annual Best Buddies Challenge event on Saturday afternoon, a fundraiser for the Best Buddies organization founded by Anthony Kennedy Shriver that helps people with intellectual disabilities. The event attracted celebrities, including New England Patriots quarterback Tom Brady and Olympian Carl Lewis.
California Gov. Arnold Schwarzenegger and his wife, Maria Shriver, Kennedy's niece, said they appreciated all the messages of care they had received for the senator.
"It's always a comfort to the family to know that Sen. Kennedy is in the prayers of millions," their statement said.
Sen. John Kerry of Massachusetts, who went to the hospital, New York Sen. Hillary Rodham Clinton and Obama said were offering their prayers for his quick recovery.
Obama, beginning a tour of hospitals in Eugene, Ore., told reporters that he had been in touch with the senator's family. He said, "We are going to be rooting for him. I insist on being optimistic about how it's going to turn out."
A man walking by Massachusetts General was startled by the news when he asked about the reason for the large media presence. "Ted? Is he all right? Jeez, I'm taken aback. I just saw him on television yesterday," said Jerry Leonard, 76.
"He's a Kennedy. His name is synonymous with this area," the retired bartender said. "I'm a Bostonian, too, and he's done a lot for us around here and for the senior citizens in particular."
Associated Press writers Lauran Neergaard in Washington, Matt Pitta in Hyannisport, Mass., and David Espo in Boston contributed to this report.