NEW YORK -- His voice is raspy, but his attitude is upbeat.

ABC news anchor Peter Jennings said he plans to continue on "World News Tonight" as much as he can despite being diagnosed with lung cancer. He'll begin chemotherapy next week.

In a videotaped message, Jennings, the network's chief anchor since 1983, told viewers "On good days, my voice will not always be like this."

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Fact Sheet: Lung Cancer Overview

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Jennings said he was surprised at how fast the news traveled and at the kindness he has received from so many people.

A former smoker who quit 20 years ago, Jennings weakened after 9/11 and started again. The 66-year-old anchor was too ill to work Saturday during the network's special report on Pope John Paul's death.

Fellow anchor Tom Brokaw said Tuesday he was "heartbroken" at the news his friend Peter Jennings has been diagnosed with lung cancer.

Jennings is a "tough guy," the former NBC News anchor said, adding he's "counting on him getting through this very difficult passage."

Jennings, the chief ABC News anchorman for more than 20 years, announced his diagnosis Tuesday morning. A network spokesman said he will begin outpatient treatment next week.

Jennings has anchored "World News Tonight" for more than 20 years. Dan Rather and Brokaw joined him in the long-running trinity of anchors, until both stepped away from the desk in the past several months.

ABC News President David Westin said that both he and Jennings expect that Jennings will continue to anchor "World News Tonight" during the period of treatment "to the extent he can do so comfortably."

Charlie Gibson, Elizabeth Vargas and others will be substituting for Jennings as necessary and when their other responsibilities permit, according to ABC News.

Westin's memo to ABC News staffs nationwide said, "All of us at ABC News have watched over the years as Peter has led us on various assignments with strength and with courage. We've done our best to support him in these endeavors. Now this. Peter's been given a tough assignment."

Jennings sent an e-mail to his staff saying:

Dear All - Forgive me the group mailing -- but it seems the easiest way to tell a lot of people I care for ... about a change in my life.
I have been diagnosed with lung cancer. Yes, it was quite a surprise.
As you all know, this is a challenge. I begin chemotherapy next week. I will continue to do the broadcast. There will be good days and bad, which means that some days I may be cranky and some days really cranky! Almost 10 million Americans are living with cancer. I am sure I will learn from them how to cope with the facts of life that none of us anticipated. In all the years I have worked here I have had the most outstanding support from this news division.
Hundreds of you have been like family. It feels good to have such a family right now.
Peter

Jennings has reported several significant historical events. Jennings was in Berlin in the 1960s when the Berlin Wall was going up and there in the 1990s when it came down. He also covered the civil rights movement in the Southern United States during the 1960s and the struggle for equality in South Africa during the 1970s and 1980s.

His biography on ABC.com said that Jennings was also there when the Voting Rights Act was signed in 1965 and on the other side of the world when black South Africans voted for the first time. Jennings has worked in every European nation that once was behind the Iron Curtain.

Jennings joined ABC in 1964, briefly serving as the anchor of the ABC Evening News from 1965 to 1968.

Prior to his current appointment, Jennings served as chief foreign correspondent for ABC News and was the foreign desk anchor for World News Tonight from 1978 to 1983. Jennings also served as ABC's bureau chief for Beirut, Lebanon, for seven years.

Born in Toronto, Jennings became anchor and senior editor of World News Tonight in 1983 -- he was the youngest network anchor in TV history. He recently became a naturalized U.S. citizen.