A quantitative GLC-mass spectrometry assay was developed for the determination of the tricyclic antidepressant amitriptyline and its desmethyl metabolite (nortriptyline) in human plasma. The assay utilizes selective ion detection to monitor in a GLC effluent the MH+ molecular ions of amitriptyline and nortriptyline generated by isobutane chemical ionization. The procedure, which utilizes deuterated analogs of amitriptyline and nortriptyline as internal standards, requires 1 ml of plasma and can measure 1 ng/ml of amitriptyline and 0.5 ng/ml of nortriptyline. The curves relating the amounts of amitriptyline and nortriptyline added versus the amounts found over a 100-fold range of amitriptyline and nortriptyline concentrations are straight lines with intercepts of approximately zero and slopes of unity. Analyses of plasma samples from three subjects receiving 50 mg of amitriptyline orally, three times a day, gave an average plasma concentration of 115 +/- 42 ng/ml for amitriptyline and 109 +/- 20 ng/ml for nortriptyline. Similar analyses of the plasma of three subjects who had received a single 50-mg oral dose of amitriptyline showed an average maximum plasma concentration of 25 +/- 10 ng/ml for amitriptyline and 10 +/- 4 ng/ml for nortriptyline. Seventy-two hours after adminis-ration, the average plasma amitriptyline and nortriptyline levels were 3 +/- 2 ng/ml, respectively.