Localization of the binding sites for a radioactive form of the tricyclic antidepressant imipramine has been accomplished using receptor autoradiographic techniques. These studies demonstrated a marked overlap between regions presenting dense concentrations of imipramine binding sites and regions known to receive serotonergic projections. Lesions of the serotonergic neurons were created by intracerebroventricular injections of the potent neurotoxin 5,7-dihydroxytryptamine. The effectiveness of these lesions was ascertained by immunohistochemical localization of serotonin containing neurons in both lesioned and vehicle treated control animals. Animals lesioned in this fashion, showed a dramatic loss of imipramine binding sites in all of the areas which were previously densely labeled. The catecholamine containing neurons were lesioned in another group of animals using 6-hydroxydopamine. The effects of these lesions were ascertained using fluorescence histochemical techniques. In these animals, the imipramine binding sites were unaltered. These results suggest that imipramine binding sites are presynaptic and are located on serotonin containing terminals and not on catecholamine containing neurons.