The ability of fluoxetine, norfluoxetine, sertraline and desmethyl sertraline to inhibit the CYP3A subfamily of cytochromes P450 was examined in vitro, using the formation of 1'-hydroxy midazolam as a probe for CYP3A catalytic activity. The inhibition observed with these four compounds was modeled using competitive, noncompetitive, uncompetitive and mixed competitive/noncompetitive relationships by nonlinear regression analysis. The best fit model of the inhibition of CYP3A-mediated 1'-hydroxy midazolam formation by all four compounds examined was determined to be mixed inhibition. The calculated Ki values were 65.7 +/- 12.0 microM for fluoxetine, 19.1 +/- 1.9 microM for norfluoxetine, 64.4 +/- 11.6 microM for sertraline and 48.1 +/- 11.6 microM for desmethyl sertraline. Steady-state plasma levels of fluoxetine and norfluoxetine can approach a concentration of 1 microM (approximately 350 ng/ml of plasma). Assuming an inhibitor concentration of 1 microM and a concentration of the substrate substantially below its Km (at least 10-fold lower), the results reported predict that fluoxetine and norfluoxetine together would inhibit CYP3A catalytic activity by less than 7% (less than 0.7% if the unbound plasma concentration of fluoxetine is considered). By using the same assumptions and concentrations for sertraline and desmethyl sertraline, these agents together would be predicted to inhibit the metabolic clearance of a coadministered CYP3A metabolized drug by less than 4%. The observations reported here suggest that fluoxetine and sertraline would have little effect on CYP3A-mediated clearance of coadministered drugs.