Dietary butylated hydroxytoluene (BHT) fed 14 days before and 14 days after carcinogen administration resulted in a dose-dependent inhibition of 7, 12-dimethylbenz[a]anthracene (DMBA)-induced mammary tumor incidence in outbred Sprague-Dawley rats. In addition, the inhibitory effects of BHT were strongly influenced by the dose of initiating carcinogen and the type of diet in which BHT was administered. In animals fed the NIH-07 diet and receiving a low dose of DMBA (5 mg/rat), the inhibitory effect of BHT was manifested at all four BHT concentrations (6,000 leads to 300 ppm). Maximal inhibition was approximately 50% in animals given 5 mg DMBA and receiving 6,000 ppm BHT. However, in the group administered a high dose of DMBA (15 mg/rat), the inhibitory effect of BHT was expressed only at 6,000 ppm, the highest concentration given. Lower concentrations (300 and 1,000 ppm) of BHT had no detectable effect on tumor incidence. In animals fed the defined, semipurified AIN-76A diet during the 4-week treatment period and initiated with 5 mg DMBA, BHT at 6,000 ppm inhibited tumor development. However, at 15 mg DMBA animals fed the AIN-76A diet differed markedly from those fed the NIH-07 diet. In the former group, BHT at 6,000 ppm was unable to elicit any inhibitory response; in the latter group, BHT inhibited tumor development by 40%. Dietary BHT also inhibited DMBA-induced adrenocortical hyperplastic nodules in a dose-dependent fashion. These results indicate that short-term exposure to dietary BHT can inhibit experimental mammary tumor development at environmentally relevant concentrations.