The politician, 76, suffered an apparent seizure, his family said.
"He is undergoing a battery of tests at Massachusetts General Hospital to determine the cause of the seizure. Sen. Kennedy is resting comfortably, and it is unlikely we will know anything more for the next 48 hours," a statement from his office said.
"He was taken to Cape Cod Hospital briefly this morning and then he was transferred to a Boston hospital," confirms Cape Cod Hospital spokesperson David Riley, who wouldn't comment on Kennedy's symptoms.
In October, Kennedy underwent surgery to clear his carotid artery but recovered quickly.
The senator, whose brother was late President John F. Kennedy, is one of only six in history to serve in the Senate for more than 40 years.
– Molly Lopez