We sought a method for routine therapeutic monitoring of serum tricyclic antidepressants (TCAs) which offered good reproduc-ibility, detection limits, linearity, and specificity, and which was simple, rapid, and inexpensive to perform. The method described utilizes Clin-ElutTM columns (Analytichem International, Inc., Lawndale, CA 90206) to facilitate the extraction. The analysis is by high pressure liquid chromatography (HPLC) with a CN bonded phase column, a mobile phase of acetonitrile/pH 7.0 phosphate/methanol and detection at 210 nm. This chromatographic system gives short equilibration times, stable calibration curves, high sensitivity and resolution, short retention times, and long column life. The method is useful for determination of amitriptyline, doxepin, imipramine, nortriptyline, nordox-epin, desipramine, and protriptyline. Trimipramine is used as an internal standard for the tertiary amines and protriptyline for the secondary amines. Recovery is linear from 25 to 1,000 ng/ml. Rubber stoppers of a new formulation in VacutainerTM blood collection tubes (Becton-Dickinson, Rutherford, NJ 07070) do not affect serum TCA levels. Sera from 53 psychiatric patients suffering from endogenous depression were analyzed using the procedure presented. The mean serum level of four patients on amitriptyline therapy having complete remission was 201 ng/ml (range, 123–259). The mean serum level of four patients on imipramine therapy rated as having complete remission was 200 ng/ml (range, 145–258). These values compare well with recently published therapeutic ranges.