Biotransformation of the H-1 antagonist terfenadine to its desalkyl and hydroxy metabolites was studied in vitro using microsomal preparations of human liver. These metabolic reactions are presumed to be mediated by Cytochrome P450-3A isoforms. The azole antifungal agent ketoconazole was a highly potent inhibitor of both reactions, having mean inhibition constants (K1) of 0.037 and 0.34 micro Meter for desakyl- and hydroxy-terfenadine formation, respectively. Itraconazole also was a potent inhibitor, with K1 values of 0.28 and 2.05 micro Meter, respectively. Fluconazole, on the other hand, was a weak inhibitor. Six selective serotonin reuptake inhibitor antidepressants tested in this system were at least 20 times less potent inhibitors of terfenadine metabolism than was ketoconazole. An in vitro-in vivo scaling model used in vitro K1 values, typical clinically relevant plasma concentrations of inhibitors, and presumed liver:plasma partition ratios to predict the degree of terfenadine clearance impairment during coadministration of terfenadine with these inhibitors in humans. The model predicted a large and potentially hazardous impairment of terfenadine clearance by ketoconazole and, to a slightly lesser extent, by itraconazole. However, fluconazole and the six selective serotonin reuptake inhibitors (SSRIs) at usual clinical doses were not predicted to impair terfenadine clearance to a degree that would be of clinical importance. Caution is nonetheless warranted with the coadministration of SSRIs and terfenadine when high doses of SSRIs (particularly fluoxetine) are administered. Also, some individuals may be unusually susceptible to metabolic inhibition for a variety of reasons. (J Clin Psychopharmacol 1996;16:104-112).