ORURO, Bolivia — President Hugo Chávez of Venezuela announced Saturday in Caracas that he would have to undergo another operation for cancer, and he designated his vice president, Nicolás Maduro, as his successor if he should prove unable to continue to lead the country.

Mr. Chávez, appearing somber and contemplative, made the announcement in a televised address from the presidential palace. Mr. Maduro sat to his left, and several other cabinet members were also present.

It was the first time that Mr. Chávez had said publicly whom he wanted as his successor. Mr. Chávez said that he would fly to Havana on Sunday for the operation. The announcement came just weeks after he was elected to a new six-year term, beginning in early January.

He said Saturday that tests immediately after his re-election found no cancer. But he said he later experienced swelling and pain. He went to Cuba on Nov. 27 for what the government said was hyperbaric treatment meant to aid in healing.