ABC News employees learned of Jennings' condition in a surprise e-mail from news president David Westin.

Jennings - who will be 67 in July - in a letter to "World News Tonight" staff, said he was surprised at the turn of events but expressed his determination to begin chemotherapy next week and to fight the disease.

He asked the staff for patience and explained that from time to time he may get a little "cranky" but to bear with him through this.

Westin announced that "World News Tonight" will be anchored on a rotating basis by "Good Morning America's" Charlie Gibson and "20/20's" Elizabeth Vargas.

Last week Westin announced that another ABC veteran, Ted Koppel, had opted to retire from the network when his contract expires in December.

At that time, Westin told reporters that he expected Jennings to renew his contract with the network when it expires in December. In a memo to ABC staff Westin stated that he expected Peter to remain with the network "for many years to come."

It is not clear now whether Koppel, who has been rumored to be negotiating with CBS, may now re-evaluate his decision.

Jennings began his career at ABC in 1964 and became the network's first evening news anchorman in 1965.

A native of Toronto, the veteran newsman began his broadcasting career at age 9 when he "hosted" a children's show on the CBC (Canadian Broadcasting Company) titled "Peter's Place."

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