From the Boston Globe:

Doctors believe that Senator Edward M. Kennedy of Massachusetts suffered a seizure at his home in Hyannis Port this morning, then a second seizure as he was being transported by helicopter from Cape Cod Hospital to Massachusetts General Hospital in Boston, according to an official briefed on the situation.

His condition was unclear early this afternoon, as a special police security detail gathered at Mass. General. Kennedy family members were called this morning and told to rush to Boston, according to sources.