Male Wistar rats were treated daily for 7 days with clofibrate (250 mg/kg/d), benfluorex (50 mg/kg/d), tiadenol (200 mg/kg/d), nicoclonate (100 mg/kg/d) or hexanicit (50 mg/kg/d). The cytochrome P 450 level and ethoxycoumarin deethylase activity (ECDE) in liver microsomes were markedly increased by administration of clofibrate and slightly increased by tiadenol. Benfluorex only increased the activity of ECDE and nicoclonate and hexanicit had no effect. Clofibrate, tiadenol and benfluorex increased the activity of microsomal bilirubin UDP-glucuronosyltransferase. On the other hand, the nicotinic derivatives were ineffective. Tiadenol clearly enhanced the inductive effects of phenobarbital.