Antidepressants are frequently used in the treatment of depressive symptoms associated with schizophrenia. In patients taking clozapine, choice of antidepressant is complicated by additive pharmacodynamic effects and by pharmacokinetic interactions. We predicted that citalopram would not elevate plasma clozapine levels when the two drugs were co-administered because it does not inhibit the relevant enzyme systems. In this preliminary study of five patients given citalopram and clozapine there was no overall change in mean clozapine levels. Based on this limited evidence, citalopram might be the antidepressant of choice in patients taking clozapine.