Peter Jennings (search) is expected to start undergoing chemotherapy (search) Tuesday as part of his treatment for lung cancer (search), a diagnosis he emotionally revealed last week on "World News Tonight" (search).

The 66-year-old ABC News (search) anchor spent most of Monday planning Monday night's telecast in his office at the network's NYC headquarters.

Jennings has been absent from the nightly program for more than a week, but has been working behind the scenes.

He has not said exactly what kind of lung cancer he has or how far along it is.

Nor is it known when he plans to return to the air. Depending on the type of drugs administered, he may feel nauseous, weak and tired after Tuesday's treatment.

Last Wednesday, when Jennings told his audience of the dire diagnosis, he was barely able to get the words out. It remains unclear what happened to his voice.

Some doctors believe it may have been affected by a medical test in which something was inserted down his throat. Others think it could be a side effect of damage caused by the disease.

ABC News sources said Monday that Jennings had "nearly gotten his voice back."