The binding of 3H-paroxetine and 3H-imipramine has been compared in 17 different regions of 12 human control brains. Our findings reveal that the regional distribution is similar for both radioligands and their bindings tend to be parallel in the brain. The highest binding site density was found in basal ganglia (hypothalamus Bmax 780 +/- 102 fmol/mg protein for 3H-imipramine binding and Bmax 515 approximately 83 for 3H-paroxetine binding). The lowest values were found in cortical areas (cingulate cortex 191 +/- 18.5 fmol/mg for 3H-imipramine binding and 88 +/- 7.5 fmol/mg for 3H-paroxetine binding). The Kd values for 3H-paroxetine binding to neuronal membranes were similar in all brain regions (mean +/- s.d. Kd 0.07 +/- 0.007 nM) and also for 3H-imipramine binding (mean +/- s.d. Kd 1.05 +/- 0.12 nM). As these values are the same as in platelet membrane, the results obtained indicate that both binding sites are identical in neuronal and in platelet membranes. These findings suggest that both ligands are good markers of the 5HT transporter. However, the higher affinity of 3H-paroxetine confirms that this compound is a better radioligand for the 5HT uptake site.