Sen. Edward M. Kennedy of Massachusetts spent his third day at Boston’s Massachusetts General Hospital yesterday undergoing medical tests to determine the cause of a seizure he suffered Saturday morning at his home on Cape Cod.

Kennedy, 76, was surrounded by family members, including his youngest son, U.S. Rep. Patrick J. Kennedy, of Rhode Island, and the senator’s wife, Victoria Reggie Kennedy.

President Bush called yesterday to wish Kennedy well. “Take care of my friend,” Mr. Bush told Kennedy’s wife.

While Democrat Kennedy remains anathema to many Republican conservatives, Mr. Bush has hosted him socially at the White House and spoken at an education event in Boston hosted by Kennedy. Senator Kennedy and the president worked together early in Mr. Bush’s first term on education policy, particularly enactment of the No Child Left Behind law.

Kennedy was transferred to Mass. General from Cape Cod Hospital on Saturday after becoming ill after walking his dogs at his oceanfront compound in Hyannisport. The senator has undergone a battery of tests to determine what caused the seizure.

The senator was alert and speaking with family members yesterday at the hospital, said Robin Costello, spokeswoman for Patrick Kennedy.

Last October, the senator, who has served since 1962 and has never lost an election in his home state, had surgery to clear a blockage in a neck artery that is a major supplier of blood to the brain. The procedure was intended to prevent a stroke. At the time, doctors said Kennedy had a major blockage in his carotid artery.

The surgery, however did little to slow Kennedy’s pace. The Massachusetts Democrat is known as one of the Senate’s most energetic lawmakers and campaigners. In recent months, he has crisscrossed the country in support of the presidential campaign of Illinois Sen. Barack Obama and continued his drumbeat of opposition to the Iraq war.

Kennedy also keeps a busy schedule in Massachusetts. He was scheduled to appear at a charity luncheon event last Saturday on Cape Cod.

Distinguishing between a seizure and a small stroke can sometimes be difficult, said Dr. Andrew Blum, a Rhode Island Hospital neurologist and assistant clinical professor at Brown University’s Warren Alpert School of Medicine.

Among the tests Kennedy is likely to have undergone, Blum said, are CT scans, an EEG or electroencephalogram and an MRI, or magnetic resonance imaging. “This is all detective work,” said Blum, who said he was giving general information and that Kennedy is not his patient.

A stroke is a neurological event caused by a blockage in blood flow to a part of the brain. A seizure is a disruption of electrical signals in the brain, Blum said.

smackay@projo.com