To study correlations between the concentrations, in serum, of amitriptyline and its most important metabolites with clinical response in patients, we developed a "high-performance" liquid-chromatographic method for routine determination of amitriptyline, nortriptyline, total 10-hydroxy-amitriptyline, desmethylnortriptyline, and E(trans)- and Z(cis)-10-hydroxynortriptyline. These compounds are extracted from 1 mL of alkalinized serum into hexane/isoamyl alcohol (99/1 by vol). Perazine is the internal standard. To minimize irreversible adsorption of the drugs onto the glassware, 5 micrograms of maprotiline is added to the organic phase just before evaporation. After a 10-min resolution on a silica column eluted with acetonitrile/methanol/NH4OH (1 mol/L), absorbance is measured at 240 nm. Only chlorimipramine, doxepin, procainamide, and N-acetylprocainamide may interfere with assay of the compounds that probably are therapeutically relevant: amitriptyline, nortriptyline, and E-10-hydroxynortriptyline. Uremia, lipemia, and icterus also do not affect the analysis.