In chronic carcinogenic bioassays, chemicals being tested with low water solubility have been administered via corn oil gavage. The present study examined the effect of chronic corn oil gavage on hepatic tumor formation in the B6C3F1 male mouse. Mice were initiated with diethylnitrosamine (DENA) either at 15 days of age with a single i.p. injection (5 micrograms/gbw) (protocol 1) or at 4 weeks of age via the drinking water (15 mg/l) for a duration of 3 weeks (protocol 2). At weaning (protocol 1) or 8 weeks of age (protocol 2) initiated and untreated mice were administered either corn oil at a dose of 0.15 ml via gavage (once a day, 5 days/wk) or saline (0.15 ml via gavage, once a day 5 days/wk). All mice were killed at 28 weeks of age and hepatic lesions were quantitated. Only mice exposed to DENA demonstrated hepatic tumors. Mice treated with DENA (at 15 days of age) and corn oil gavage exhibited a significant decrease in the number of hepatic adenomas compared with DENA (at 15 days of age) only treated mice. No difference was noted in the number of hepatic adenomas between mice treated with DENA (at 4 wks of age) and corn oil gavage and mice exposed to DENA (at 4 wks of age) only.