Control data on F344/N rats and (C57BL/6N X C3H/HeN)F1 (B6C3F1) mammary tumor virus-free mice from the National Toxicology Program (NTP) were examined to determine if animals receiving corn oil by gavage showed tumor incidences that differed from those of untreated control animals. Analyses of these data were adjusted for interlaboratory variability, time-related trends, and supplier effects. Two biologically significant effects were found: Male F344/N control rats receiving corn oil by gavage showed a higher (P less than .05) incidence of pancreatic acinar cell adenoma and a lower (P less than .001) incidence of leukemia (primarily mononuclear cell leukemia) than did the corresponding untreated controls. The increased incidences of pancreatic acinar cell adenoma seen in male rats administered corn oil by gavage were associated with elevated body weights observed in these animals relative to untreated controls. Female F344 rats and male and female B6C3F1 mice showed little or no evidence of a difference in tumor incidence between corn oil gavage-treated and untreated controls. A review of nearly 300 carcinogenesis studies done by the National Cancer Institute (NCI) and the NTP revealed that there were no corn oil gavage studies in which increased incidences of pancreatic acinar cell tumors or leukemia in male F344/N rats were the sole evidence of the carcinogenicity of a test chemical. Thus use of corn oil appears to have little impact on the interpretation of NCI-NTP carcinogenicity studies.