A simple procedure is described that permits the simultaneous determination of trimipramine and its two major metabolites, desmethyl- and hydroxytrimipramine, in human plasma or red blood cells (RBCs) at therapeutic concentrations. The extracted biological fluids are injected into a capillary gas chromatograph with an OV-1 fused-silica column coupled to a nitrogen-phosphorus-selective detector. The limit of determination for trimipramine is 3 ng/ml and for that desmethyl- and hydroxytrimipramine is 4 ng/ml. The method permits the RBC/plasma ratios to be determined and to be correlated with the clinical response.