Overnight-fasted rats were fed 75 mg of amitriptyline (AMI). Two hours after dosing, the rats were anesthetized and blood samples were drawn from the femoral vein. The rats were sacrificed and left at room temperature for periods ranging from 0.1 to 96 hours. Samples were then obtained of heart blood and blood from the inferior vena cava, and of tissue from different liver lobes, the heart, lungs, kidneys, thigh muscle, and the brain. Samples were analyzed by high performance liquid chromatography. Results showed that AMI concentration in heart blood increased fairly rapidly within the first 2 hours postmortem; from then on, the average ratio was 6.4. In peripheral blood, the post/antemortem AMI concentration ratio followed an exponential rise; at 2 hours and 96 hours postmortem, the ratio was 1.6 and 55.1, respectively. For the main metabolite nortriptyline, concentration changes followed the same pattern but to a lesser extent. Among tissues, liver lobes had high but variable drug concentrations. Lobes closest to the stomach had the highest drug concentrations. The drug concentration in lungs declined significantly. The animal model demonstrated postmortem drug concentration changes similar to those in humans. Probable mechanisms included drug diffusion from the stomach and gastrointestinal tract to surrounding tissues and blood, as well as postmortem drug release from the lungs and possibly other drug-rich tissues into the blood. 24 references, 3 tables, and 5 figures