Dehydroepiandrosterone (DHEA) a naturally occurring steroid, has been reported to inhibit the binding of N-dimethylnitrosamine and 7,12-dimethylbenz[a]anthracene to DNA in vivo and to increase glutathione transferase activity. In this study, we have investigated if DHEA could protect hepatic DNA from damage by the potent hepatocarcinogen aflatoxin B1 (AFB1). Young male Fischer 344 (2-month-old) rats were fed a diet containing 0.8% DHEA for 14 days. Control rats were pair-fed the same diet without DHEA. The rats were then administered a single i.p. dose of [3H]AFB1 in dimethylsulfoxide (0.6 mg/kg body weight; 200 mCi/mmol) and killed after 3 h. Liver weight, mitochondrial, microsomal and cytosolic protein, cytochrome P450 content and glutathione transferase activity increased significantly (P less than 0.001) in DHEA-fed rats; however, the hepatic DNA content was not altered. DHEA feeding increased the total amount of AFB1 bound to hepatic protein but decreased the extent of DNA binding. In in vitro experiments, there was less total binding to DNA and protein by AFB1 when using microsomes from DHEA-fed rats. These results suggest that DHEA inhibits the binding of AFB1 to DNA by modifying the biotransformation of the carcinogen.