The effect of acetone pretreatment (5% in drinking water for 10 days on rat liver metabolism and toxicity of thiobenzamide (TB) was investigated. Hepatic microsomes from acetone-pretreated rats showed a significant increase of TB-S-oxidation rate which, on the basis of selective thermal inactivation of FAD-containing monooxygenase (FADM), appeared dependent only on cytochrome P-450. Furthermore, TB was able to competitively inhibit acetone hydroxylase (AcH), an enzymatic reaction highly specific for the P-450IIE1 isozyme. Acetone pretreatment of rats also produced an exacerbation of liver damage induced by acute administration of TB (150 mg/kg), as judged by the extent of liver necrosis and serum alanine-amino transferase (ALAT) activities. Coadministration of acetone with TB reduced on the other hand the extent of liver damage. The findings suggest that P-450 species induced by acetone, and in particular the P-450IIE1 isozyme, could be involved in the biotransformation of TB.