
Kennedy, the liberal lion of the Democratic Party, age 76, was taken by helicopter to Massachusetts General Hospital in Boston, broadcast reports said.
"It appears that Senator Kennedy experienced a seizure this morning," Wagoner said. "He is undergoing a battery of tests ... to determine the cause of the seizure. Senator Kennedy is resting comfortably, and it is unlikely we will know anything more for the next 48 hours."
The junior senator from Massachusetts, former presidential candidate John Kerry, and Kennedy's niece Caroline Kennedy, daughter of slain president John F Kennedy, visited him at hospital but did not speak to media afterwards.
CNN reported that Kerry stayed 45 minutes at the hospital.
According to a report by the Cape Cod Times, Kennedy was rushed by ambulance at 8:19 am from the family compound in Hyannisport, Massachusetts, to Cape Cod Hospital after falling ill.
He spent two hours in the local emergency room before the decision was made to transfer Kennedy by helicopter to the Boston hospital.
Kennedy is the last surviving brother of President John F Kennedy, who was assassinated in November 1963. Another older brother, Robert Kennedy, was assassinated in June 1968 while seeking the Democratic Party presidential nomination. Yet another, Joseph Jr, died as a US combat pilot in 1944.
News of Ted Kennedy's illness rippled through the US political world, with presidential candidates expressing concern.
Presumptive centre-right Republican candidate Senator John McCain, who has served with Kennedy for years and often been at loggerheads with the liberal giant, called his colleague a "legendary lawmaker whose role in the US Senate cannot be overstated."
Senator Hillary Clinton, still struggling to keep head above water in her own fight for the Democratic presidential nomination, referred to Kennedy's persistent fight for the poor.
"Nobody has fought harder to make sure that everybody got good healthcare," Clinton said in broadcast remarks.
The Massachusetts Senator, long an icon of the liberal wing of the Democratic Party, most recently made headlines in late January when he and niece Caroline publicly endorsed Barack Obama's bid for the Democratic Party's presidential nomination.
The move cut the rug out from under the candidacy of Clinton, who along with husband, ex-president Bill Clinton, had become close friends of the fabled clan during the 1990s.
Kennedy himself once had presidential ambitions, but he became bogged down in the legacy of the fatal 1969 accident at Chappaquiddick, Massachusetts, that claimed the life of a woman passenger in a car he drove off a bridge.
Kennedy swam to safety but the woman drowned, and Kennedy later pleaded guilty to leaving the scene of the accident.
Nonetheless, his influence remains huge on the US political scene, where he has used his 45-year seniority in the senate to protect the disenfranchised and advocate for the elderly, disabled, immigrants, workers and civil
Kennedy has served as senator from Massachusetts since 1962 when he was elected to the post vacated after his brother John became president.
He was the youngest of nine children of Joseph P Kennedy and Rose Fitzgerald Kennedy. He is a graduate of Harvard University and the University of Virginia Law School.
He makes his home in Hyannisport, with his second wife Victoria Reggie Kennedy, and her children from a previous marriage, Curran and Caroline Raclin. He had three other children with his ex-wife Joan - Kara, Edward Jr and Patrick.
Kennedy, the second longest-serving senator, is the senior Democrat on the Health, Education, Labor and Pensions Committee in the Senate and also belongs to the Judiciary Committee, where he is the senior Democrat on the Immigration Subcommittee, and on the Armed Services Committee, where he is the senior Democrat on the Sea power Subcommittee.
He is also a member of the Congressional Joint Economic Committee and the Congressional Friends of Ireland, and a trustee of the John F. Kennedy Center for the Performing Arts in Washington, DC.