This study evaluated the effects of the commonly used hydrophilic organic solvents, acetonitrile, methanol, ethanol, 1-propanol, dimethyl sulfoxide (DMSO), N,Ndimethylformamide, polyethylene glycol and propylene glycol, on CYP3A in pooled human liver microsomes, using testosterone and midazolam as substrates. Furthermore, we examined the modulation effect of organic solvents on CYP3A inhibition by ketoconazole. Testosterone 6 beta-hydroxylation activity was potently inhibited in the presence of DMSO and 1-propanol in a concentration-dependent manner. Midazolam 1'-hydroxylation activity, however, was weakly inhibited only by 1% of DMSO, the highest concentration used in this study. Moreover, the potency of ketoconazole to inhibit CYP3A activities was variable, depending on the organic solvent used as a dissolving solvent for ketoconazole. Our data indicate that each organic solvent had an effect on CYP3A4 activity, evaluated by both substrates with different magnitudes. Furthermore, it was shown that the effects of organic solvents on CYP3A activity are substratedependent. The present study also shows that methanol had little effect on either substrate.