Our aim was to investigate the inhibitory effects of schisandrin A and schisandrin B on cytochrome P450 (CYP3A) activity in rat liver microsomes and the mechanism of this interaction. 1'-Hydroxy midazolam and midazolam 1-hydroxylation catalyzed by CYP3A were analyzed by high performance liquid chromatography (HPLC). Results showed that schisandrin A and schisandrin B inhibited CYP3A activity with IC(50) values of 6.60 and 5.51 microM and K(i) values of 5.83 and 4.24 microM, respectively. A dilution assay plot of each inhibitor gave a slope value of up to 91% that of the control samples. The inactivation of CYP3A activity by schisandrin A and schisandrin B was found to be both time-and concentration-dependent (schisandrin A: K(I) = 4.51 microM, K(inact) = 0.134/min; schisandrin B: K(I) = 3.01 microM, K(inact) = 0.112/min). We conclude that the inhibition of CYP3A activity in rat liver microsomes by schisandrin A and schisandrin B is mostly attributed to a mixed noncompetitive and complete inhibition.