BOSTON — Senator Kennedy, who has brain cancer, was taken by ambulance to a hospital today near his Cape Cod vacation home after complaining of feeling ill but returned home a few hours later.

A statement issued by the 76-year-old senator’s office attributed the episode to a change in medications. Senator Kerry said his fellow Massachusetts Democrat was well and planned to watch the evening’s presidential debate between Senators McCain and Obama.

A 911 call from the Kennedy compound in Hyannis Port came around 5 p.m., Barnstable police Sergeant Ben Baxter said. Mr. Kennedy was taken to Cape Cod Hospital and was “alert and responsive” during the trip, Sergeant Baxter said.

Mr. Kennedy’s office confirmed he returned home about 8 p.m.

He had a seizure in May and underwent surgery in June for a malignant brain tumor. After undergoing six weeks of chemotherapy and radiation treatment, he has been steadily increasing his public activity.

Mr. Kennedy had told reporters visiting his Hyannis Port home on Tuesday that he was actively following the presidential race. “I’m going to be following it particularly next Friday,” the senator said with a laugh.

He received a visit that day from President Bachelet of Chile at his home in his first public appearance since he gave a surprise speech in late August at the Democratic National Convention.

Ms. Bachelet presented the senator with her country’s highest civilian award — the Order to the Merit of Chile — in recognition of his opposition to the country’s 1973 government overthrow and his work to cut off military aid to dictator Augusto Pinochet.

At the meeting, the senator stumbled momentarily as he tried to walk on the thick grass, but he quickly regained his balance. While he didn’t take questions from reporters, he departed from his prepared text to point out landmarks to Bachelet and tell a story about a friend in the crowd.

“There’s a wonderful relationship between our family and the sea that goes back a long time,” he said as he pointed to Nantucket Sound and the mooring where his schooner “Mya” rocked in the waves.

In early September, Mr. Kennedy announced he would not return to the Senate until January, but he has since held videoconferences with his staff and members of the Senate Committee on Health, Education, Labor and Pensions, which he leads.

Mr. Kennedy surprised his colleagues in July when he made a surprise return to Capitol Hill to vote on a Medicare bill.