The pharmacological action of cyclopyrrolones, zopiclone(ZPC) and suriclone(SRC) have been shown to be very similar to the benzodiazepines, although cyclopyrrolones have basic molecular frameworks distinct from those of benzodiazepines, Clinically, minor tranquilizers are used for a long period. Therefore, we have studied the effects of ZPC and SRC on rat liver microsomal drug metabolizing system. ZPC and diazepam (DZP) were administrated for 5 days (100mg/kg p.o.). The contents of cytochrome P-450(P-450) and cytochrome b5 (b5) and the activities of NADPH-cytochrome c reductase(reductase), aniline(AN) p-hydroxylase and aminopyrine(AM) N-demethylase were increased in DZP treated rats. On the other hand, ZPC induced the activities of reductase and AN p-hydroxylase to the same extent as DZP, but not the contents of P-450 and b5 and AM N-demethylase activity. The AN p-hydroxylase activity and b5. content were enhanced by the administration of SRC (60 mg/kg p.o., twice a day for 3 days), but the AM N-demethylase activity and P-450 content were not changed. Furthermore, no influence on microsomal drug metabolizing system was observed when SRC (0.7 5mg/kg p.o.) was administrated for 14 days. From these results, it was suggested that ZPC and SRC had inducing effects on rat liver microsomal drug metabolizing system, but these effects were less than DZP.