Repeated treatment with imipramine (10 mg/kg intraperitoneally (i.p.), once daily for 14 days) caused a decrease in the Bmax, without affecting the Kd, of [3H]DTG (1,3-di-o-tolylguanidine) binding to the haloperidol-sensitive sigma sites in the striatum, hippocampus and cerebral cortex of the rat. A similar reduction was observed after chronic administration of a selective serotonin uptake inhibitor, fluoxetine (10 mg/kg i.p., twice daily for 14 days), but not of a selective norepinephrine uptake inhibitor, desipramine (10 mg/kg i.p., once daily for 14 days). Neither a single injection of imipramine (10 mg/kg i.p.) nor addition of imipramine or fluoxetine into the binding assay medium mimicked the changes in the maximal binding of brain sigma sites induced by chronic treatment with these drugs. Finally, depletion of brain serotonin by means of repeated administration of p-chlorophenylalanine, which produces inhibition of the amine synthesis, blocked the ability of repeated imipramine treatment to reduce the maximal number of [3H]DTG binding sites in the striatum and hippocampus. The present results suggest that cerebral serotonergic transmission may play a role in the regulation of cerebral sigma binding sites in the rat.