OBJECTIVE The aim of the study was to characterize further the role of CYP3A4 in the metabolism of tricyclic antidepressants. METHODS The effect of oral ketoconazole (200 mg day-1 for 14 days) on the kinetics of a single oral dose of imipramine (100 mg) and desipramine (100 mg) was evaluated in two groups of six healthy male subjects. RESULTS Ketoconazole administration was associated with a decrease in imipramine apparent oral clearance (from 1.16 +/- 0.21 to 0.96 +/- 0.20 l h-1 kg-1, mean +/- s.d.; P < 0.02), a prolongation in imipramine half-life (from 16.7 +/- 3.3 to 19.2 +/- 5.4 h, P < 0.05) and a decrease in area under the curve of metabolically derived desipramine (from 3507 +/- 1707 to 3180 +/- 1505 nmol l-1 h, P < 0.05), whereas concentrations of 2-hydroxy-imipramine were unaffected. In the subjects given desipramine, no significant changes in desipramine and 2-hydroxy-desipramine kinetics were observed during ketoconazole treatment. CONCLUSIONS These findings indicate that ketoconazole, a relatively specific inhibitor of CYP3A4, inhibits the N-demethylation of imipramine without affecting the 2-hydroxylation of imipramine and desipramine. This interaction, confirms that CYP3A4 plays a role in the demethylation of tricyclic antidepressants.