1. A gas-chromatographic procedure for the estimation of amitriptyline and its metabolites in serum and urine using a nitrogen-specific detector is described. Specially cleaned glassware and purified solvents are used for the extraction of serum to further minimize extraneous peaks. Trimethylamine is added to serum before extraction to improve the recovery of drugs. Urine is refluxed at pH approximately 1 to hydrolyze the conjugates and to convert hydroxymetabolites to corresponding dehydro compounds. Serum is not hydrolyzed. 2. Two internal standards, one a tertiary amine similar in structure to amitriptyline and the other a secondary amine similar in structure to nortriptyline, are added to the specimen prior to extraction to obviate the need for accurate measurements of volumes during extraction and analysis. Urine and serum are washed with organic solvents at acidic pH to remove neutral and acidic impurities. Secondary bases are converted to their acetyl derivatives. 3. In the serum of a patient who is on amitriptyline therapy or who has ingested an overdose of amitriptyline, nortriptyline, a pharmacologically active metabolite is also measured. However, detection or estimation of hydroxymetabolites in serum is not clinically relevant. Hydroxylation index of an individual patient is determined by measuring the ratio of nortriptyline to its hydroxymetabolite in urine. 4. Amitriptyline and nortriptyline can be estimated in serum at a lower level of 10 and 20 ng/ml respectively. The procedure is linear over a wide range of amitriptyline and its metabolites. The use of an electronic integrator allows the estimation of different compounds with 100 fold difference in their concentration from the same chromatogram.