The effect of the selective serotonin reuptake inhibitor fluvoxamine (100 mg/day for 10 consecutive days) on the kinetics of a single oral dose of imipramine (50 mg) and desipramine (100 mg) was investigated in 12 healthy subjects. Compared with a control session, treatment with fluvoxamine caused a significant prolongation of imipramine half-life (from 22.8 +/- 6.4 to 40.5 +/- 5.0 h, means +/- SD, p < 0.01) and a marked decrease in imipramine apparent oral clearance (from 1.02 +/- 0.19 to 0.28 +/- 0.06 L/h/kg, p < 0.0001). No significant changes in desipramine kinetics were observed during fluvoxamine treatment. These findings indicate that, at the dosage tested, fluvoxamine markedly inhibits the demethylation of imipramine without affecting significantly the CYP2D6-mediated hydroxylation of desipramine.