The effect of oral administration of ethanol on the tumorigenicity of N-nitrosopyrrolidine (NPYR) in chow-fed male Syrian golden hamsters has been investigated. Groups of hamsters were given tap water, 7.4% ethanol or 18.5% ethanol-water mixtures 4 weeks prior to and throughout the carcinogen administration period. A total dose of 1 mmol NPYR was administered by intraperitoneal injection to both tap water and ethanol-consuming animals over a 25-week period. Twenty-four hours after the last injection, ethanol-consuming animals were returned to tap water. In control animals treated with NPYR we observed 3 26 animals with tracheal papillomas, and 6 26 animals with hepatic neoplastic nodules. In animals receiving 7.4% ethanol-water mixtures, we observed 6 26 tracheal papillomas, and 17 26 hepatic neoplastic nodules. Similar results were observed in animals receiving 18.5% ethanol. These data indicate that chronic administration of ethanol to chow-fed hamsters increases the incidence of hepatic neoplastic nodules although no differences in the two levels of ethanol were observed.