[3H]Imipramine binding was studied in the prefrontal cortex and putamen of post-mortem brains from control and Parkinsonian subjects. Saturation and inhibition curves showed both high affinity [3H]imipramine binding related to the serotonin uptake mechanism and low affinity binding which was sodium-independent and unrelated to serotonergic uptake. After subcellular fractionation, high affinity [3H]imipramine binding sites were enriched in synaptosomal fractions. In Parkinson's disease, where brain serotonin concentrations are decreased, there was a significant reduction in the density of the high affinity binding in the prefrontal cortex and putamen while the characteristics of the low affinity binding sites remained unchanged. After subcellular fractionation of the putamen of Parkinsonian patients, the decrease in [3H]imipramine binding was found predominantly in the synaptosomal fractions. These results are consistent with a relation between the high affinity [3H]imipramine binding sites and the neuronal serotonin uptake mechanism. Estimation of [3H]imipramine binding could be used as a specific marker for the study of serotonergic innervation in human post-mortem material. The reduction in the density of tricyclic antidepressant binding sites found in cortical and subcortical areas of Parkinsonian brains may be somehow implicated in the depression often seen in patients.