Previous studies have indicated that CYP3As are involved in the metabolism of the prokinetic agent domperidone. The objectives of our study were to characterize further the role of specific CYP3A isoforms in the metabolism of domperidone and to compare the kinetic parameters of domperidone to those of the CYP3A probe drug midazolam. Intrinsic clearance for the formation of domperidone major metabolite (5-hydroxydomperidone) was the highest with rCYP3A4 (0.4 mL/min/nmol CYP450) compared to rCYP3A5 (0.04 mL/min/nmol CYP450). The addition of cytochrome b5 to recombinant enzymes (rCYP3A4 or rCYP3A5) increased up to 6-fold the Vmax for the formation of 5- hydroxydomperidone. In contrast, much similar intrinsic clearance values for rCYP3A4 and rCYP3A5 were determined in the respective formation of either 1-hydroxmidazolam (1.28 and 1.57 mL/min/nmol CYP450) or 4-hydroxymidazolam (0.04 and 0.06 mL/min/nmol CYP450). Vmax for the formation of midazolam metabolites was increased to a lesser extent (1.5-3-fold) by the addition of cytochrome b5. Ketoconazole more potently inhibited CYP3A4 than CYP3A5 for both domperidone and midazolam. However, the addition of cytochrome b5 to the incubation mixture appeared to decrease the inhibitory potency of ketoconazole towards CYP3A4 for domperidone but not for midazolam. Our results indicate that CYP3A4 plays major role in the metabolism of domperidone. We demonstrated a modulatory role of cytochrome b5 mostly for the metabolism of domperidone and confirmed selective inhibition of CYP3A4 over CYP3A5 by ketoconazole. Comparison of domperidone kinetic parameters to those of the CYP3A probe drug midazolam suggests that domperidone exhibits a much higher CYP3A4/CYP3A5 selectivity ratio than midazolam.