Opioid agonists having different affinity for delta and mu receptors were injected bilaterally in the substantia nigra (SN) of rats. The selective agonist of mu receptors N-MePhe3,-D-Pro4 morphiceptin (PLO 17) produced a stereotyped behavior characterized by stereotyped sniffing and gnawing antagonized by the irreversible antagonist of mu receptors beta-funaltrexamine. In contrast, bilateral intranigral injection of the selective delta agonist D-Pen2,D-Pen5 enkephalin (DPDPE) elicited dose-dependent exploratory behavior and rearing but failed to produce gnawing. The behavioral syndrome induced by DPDPE was significantly reduced by the selective delta antagonist ICI 174,864. Naloxine, a non-selective opioid antagonist, antagonized the effects of both compounds. SCH 23390 and haloperidol, two antagonists of dopaminergic D1 and D2 receptors, respectively, blocked the effects of PLO 17 and DPDPE. The results indicate that stimulation of specific opioid receptor types in the SN elicits specific behavioral syndromes and suggest that the SN might be the site of origin of certain items of the behavioral syndrome evoked by systemic opiates. These items might be mediated by activation of dopaminergic neurons of the ventral mesencephalon.