Stereoselectivity in the oxidative metabolism of propafenone (PPF) enantiomers to 5-hydroxypropafenone was studied with untreated and inducer-treated mouse hepatic microsomes. In the untreated microsomes, Lineweaver-Burk plots of the 5-hydroxylation of R(-)- and S(+) PPF. single lines, and the intrinsic clearance (Vmax/Km) value of R(-)-PPF was 1.3-fold higher than that of S(+)-PPF. When racemic PPF was used as a substrate, the plot was shifted to the upper region, in comparison with that estimated from the sum of the individual 5-hydroxylase activities of each enantiomer, suggesting enantiomer/enantiomer interaction. In phenobarbital-induced microsomes, the Lineweaver-Burk plot for R(-)-PPF was a single line, but that for S(+)-PPF was biphasic. When racemic PPF was used as a substrate, the plot was biphasic and was shifted to the upper region in comparison with that estimated from the sum of the individual 5-hydroxylase activities of each enantiomer. The observed value of intrinsic clearance of the PPF racemate at lower concentrations (Vmax/Km1) was consistent with the estimated value, suggesting no interaction between R(-)- and S(+)-PPF. These findings indicate that most of the 5-hydroxylation of R(-)- and S(+)-PPF is catalyzed by common cytochrome P-450 species, but a part of S(+)-PFF 5-hydroxylation is catalyzed by another phenobarbital inducible cytochrome P-450 species, particularly at lower substrate concentrations.