Sen. Edward M. Kennedy, hospitalized Saturday after apparently suffering a seizure at his home on Cape Cod, Mass., was awake and joking with family members later in the day, a spokeswoman said.

The Democratic senator is undergoing tests at Massachusetts General Hospital in Boston to determine the cause of the seizure.

The 76-year-old Kennedy, leader of a storied political dynasty and a liberal icon, was rushed from the family compound at Hyannis Port, Mass., to Cape Cod Hospital at 9 a.m. He was evaluated there, then airlifted to Massachusetts General.

Kennedy suffered what first appeared to be "stroke-like symptoms," a Democratic Party aide said. The longtime senator experienced one seizure in Cape Cod and a second while aboard the helicopter flight to Boston, the Boston Globe reported.

By the end of the day, however, Kennedy was "conscious, talking, joking with family," said Kennedy's spokeswoman, Stephanie Cutter.

Nonetheless, news of his illness sent shudders through the Democratic establishment and commanded wide national attention, in part because he so vividly embodies the Kennedy legacy, with even his voice and appearance potently reminiscent of his two slain brothers.

Family members said they remained "guardedly optimistic" that he would recover soon, and hospital officials said he was resting comfortably. Relatives gathered at the hospital, joined by Kennedy's Massachusetts colleague, Sen. John F. Kerry.

The hospital canceled plans for a news briefing. Experts said a seizure is caused by the abnormal firing of neurons in the brain, producing an excess of electrical activity.

Although frequently thought to lead to a loss of consciousness or convulsions, seizures can produce symptoms as mild as numbness, nausea or a sensation of fear. Many of those symptoms also are associated with stroke, making it difficult at times to distinguish between the two.

Nearly everybody is at some risk of seizures, according to Dr. Marc R. Nuwer of UCLA's David Geffen School of Medicine. They can be triggered by sleep deprivation, stress, alcohol consumption or medications.

It is unlikely that Kennedy will suffer any long-lasting aftereffects. But Nuwer said he would probably have a headache and a sore body for several days, "like he ran a marathon."

Kennedy was elected to the Senate in 1962 to fill the seat vacated when his brother, John F. Kennedy, won the presidency two years before, and he has been a key player in U.S. politics ever since. He has figured prominently in this year's Democratic presidential race between Hillary Rodham Clinton and Barack Obama with his endorsement of the first-term senator from Illinois.

Obama, campaigning Saturday in Eugene, Ore., praised Kennedy during an appearance at a hospital.

"Ted Kennedy is a giant in American political history," Obama said. "He has done more for the healthcare of others than just about anybody in history, and so we are going to be rooting for him, and I insist on being optimistic about how it's going to turn out."

Clinton and likely GOP nominee John McCain, also Senate colleagues, likewise issued statements praising Kennedy and wishing him well.

Kennedy underwent surgery in October to clear his left carotid artery to reduce the likelihood of a stroke, and his colleagues and family members said he recovered well and soon returned to work.

He also has suffered from years of chronic back pain, dating to a plane crash in 1964.

Two of Kennedy's brothers, President Kennedy and New York Sen. Robert F. Kennedy, were assassinated during the 1960s. His oldest brother, Joseph Jr., died in World War II.

Kennedy has been a tireless campaigner on social issues such as increases in the minimum wage and improvements in healthcare.