The relationship between amitriptyline (AT) metabolism and clinical response was studied in 14 outpatients treated with a daily dose of 150 mg AT. Riboflavin was added to the medication to check compliance. On days 0, 2, and 7 and at 3, 6, 9, and 13 wk after onset of therapy, blood samples were drawn from the patients 3 (+/- 0.5) hr after the first morning dose and a sample of the first morning urine was taken to check riboflavin. Serum levels of AT and its metabolites, nortriptyline (NT), E- and Z-10-hydroxynortriptyline (E- and Z-10-OH-NT), total (E + Z) 10-hydroxyamitriptyline (tot-10-OH-AT), and desmethylnortriptyline (DNT), were measured by means of HPLC while minimizing adsorption onto glass. On day 0 and after 6 and 13 wk the severity of the depressive disorder was scored by means of the self-rating depression scale of Zung.28 Mean steady-state concentrations of AT, NT, and E-10-OH-NT were in the order of 100 micrograms/l and tot-10-OH-AT and Z-10-OH-NT approximated 20 micrograms/l. DNT concentrations were under 15 micrograms/l. There was great variation in metabolic pattern between patients. After 6 wk concentrations of all compounds were approximately 15% lower than at 3 wk, indicating a weak autoinducible effect of AT or its metabolites. Steady-state concentrations of AT correlated well with that of NT (r = 0.64; P less than 0.05) but not with that of E-10-OH-NT.(ABSTRACT TRUNCATED AT 250 WORDS)