The effect of fluvoxamine maleate, 100 mg/day for 10 days, on plasma concentrations of tricyclic antidepressants was studied in 15 depressed patients on maintenance therapy with imipramine (7 pts.) or desipramine (8 pts.). In the subgroup treated with imipramine, plasma levels of imipramine increased significantly (p < 0.001) during fluvoxamine coadministration, while levels of desipramine were not modified. Addition of fluvoxamine to patients on a stable desipramine dosage regimen resulted in a slight, but statistically not significant, increase in desipramine plasma concentrations. These results suggest that fluvoxamine is a potent inhibitor of imipramine demethylation, while it has a weak effect on the hydroxylation of desipramine.