The authors examined the effect of neuroleptic drugs on desipramine steady-state plasma concentration in 30 patients who received similar milligram per kilogram doses of desipramine. Fifteen of these patients also received a neuroloeptic drug and had desipramine plasma levels twice that of the 15 patients who received dispiramine alone. The authros discuss the implications of the magnitude of this difference for therapeutic response and incidence of side effects.