Morphologic changes induced by thio-TEPA were examined in animals with chemically induced neoplasms. Both toxic and metabolic effects were documented. Degeneration, vacuolization and increased exfoliation (toxic) were most prominent, occurred within 48 hours of exposure, and tended to subside after removal of the drug. Nuclear changes (metabolic) were rare and occurred late. Neither toxic nor metabolic effects were specific for thio-TEPA: both could be detected in control animals receiving saline. The metabolic effects of thio-TEPA are not sufficiently widespread to prevent tumor growth.