Male Wistar rats were treated with tiadenol, 200 mg/kg/ p.o. daily for 7 days. 2. The administration of tiadenol markedly increased the activity of bilirubin glucuronosyl transferase in hepatic microsomes. Cytochrome P 450 levels and ethoxycoumarin deethylase activity were more slightly increased. 3. Electrophoresis of microsomal proteins showed the induction of a protein with a molecular weight of about 72,000. 4. Tiadenol alone only slightly affected the fatty acid composition of microsomal lecithins and cephalins. When given with phenobarbital, however, tiadenol modified the phenobarbital-induced changes in the fatty acid composition of microsomal lipids.