NEW YORK - American International Group, the world's largest insurance company, said Tuesday that chief operating officer Evan G. Greenberg, his father's heir apparent as chairman and chief executive officer, had resigned.

No reasons were given, and the younger man's plans were not announced. M.R. Greenberg, who has led the company for 32 years, announced last year that Evan Greenberg would succeed him.

"We were very surprised" by the departure, said Jay Cohen, an analyst at Merrill Lynch. "There are no visible problems at the company."

AIG's stock rose TK to TK on the announcement. Mr. Cohen said the departure might mean that the elder Mr. Greenberg, who is 75, would forestall his retirement for a few more years. That could be encouraging to investors, Mr. Cohen said.

Another son, Jeffrey W. Greenberg, left the company in 1995 after speculation that he would succeed their father; he is now president and chief executive officer of New York-based Marsh & McLennan Cos., the insurance broker and owner of Putnam Investments.

AIG said Evan Greenberg's responsibilities would be reassigned to other senior executives.

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