Monocrotaline (MCT), a hepatotoxic/hepatocarcinogenic pyrrolizidine alkaloid (PA) induced DNA-DNA interstrand crosslinks in a dose-dependent manner through 30 mg/kg. Hepatic cytochrome P-450 has been shown to bioactivate MCT to pyrrole derivatives which are thought to be responsible for these genotoxic lesions. We have hypothesized these lesions to be related to the adverse hepatic actions of MCT and other PAs. Studies reported here investigated the effect of phenobarbital, a P-450 inducer, 2-dimethylaminoethyl-2,2-diphenylvalerate, a P-450 inhibitor and butylated hydroxyanisole, a dietary antioxidant, on hepatic DNA-DNA interstrand cross-links induced by a single dose of MCT (15 mg/kg i.p.) administered to male Sprague-Dawley rats. DNA damage was assessed by alkaline elution. The effects of these pretreatment regimens on MCT-induced DNA-DNA interstrand cross-linking was qualitatively similar to their reported effects on the hepatotoxicity of MCT. The effects of these pretreatments on hepatic cytochrome P-450 content, hepatic non-protein sulfhydryl levels and hepatic glutathione S-transferase activities were similarly investigated in attempts to explain the observed effects on DNA cross-link induction. These data provide further support for the association between DNA damage and the adverse hepatic effects of MCT.