Bidirectional drug interactions between fluvoxamine and classical antidepressants were studied in depressed patients. A column switching technique combined with high performance liquid chromatography (HPLC) enabled automated analyses of plasma for simultaneous determination of fluvoxamine, tricyclic and tetracyclic antidepressants and demethylated and major hydroxylated metabolites in a single HPLC run. The measurements revealed that fluvoxamine inhibited N-demethylation of imipramine, clomipramine, amitriptyline and maprotiline whereas interferences with hydroxylation reactions were restricted to aromatic 8-hydroxylation of clomipramine. In patients under fluvoxamine monotherapy before comedication, plasma concentrations of fluvoxamine increased after administration of a tricyclic antidepressant, thus indicating bidirectional drug interactions. The inhibitory effects of fluvoxamine on the metabolism of classical antidepressants disappeared after discontinuation of concomitant fluvoxamine treatment within at least 1-2 weeks. The reported alterations in drug metabolism observed in depressed patients who were under fluvoxamine/tricyclic antidepressant comedication suggested that careful supervision and regular drug monitoring are necessary in such patients.