Chronic imipramine administration decreases the density of serotonin S2 and beta-adrenergic receptors. The mode of action is not clear but might be assumed to be a result of imipramine interacting with its high affinity binding sites on presynaptic serotonin neurons. To investigate this possibility and the role of the serotonergic and noradrenergic systems on imipramine-induced receptor down-regulations, the effects of raphe lesions and 6-hydroxydopamine lesions were examined. The theory that imipramine acts through the serotonin system to down-regulate beta-adrenergic receptors was supported by the observation that raphe lesions attenuated the imipramine-induced beta receptor denumeration. However, the effect of imipramine on the S2 site may not be a result of serotonin uptake blockade. Raphe lesions which diminish 3H-imipramine high affinity binding did not alter the imipramine-induced decrease in S2 receptor numbers. 6-Hydroxydopamine lesions prevented the reduction in beta but not S2 binding. Imipramine may exert its effect on S2 binding through a site other than the high affinity, serotonin uptake site.