To examine the role of delta-opioid receptors in the regulation of the sympathoadrenomedullary system, the effects of the highly selective delta-opioid receptor agonist deltorphin (DT) on plasma catecholamine responses to insulin-induced hypoglycemia (IIH) and cold pressor test (CPT) have been investigated in normal subjects in two separate studies. DT failed to modify basal plasma levels of both norepinephrine (NE) and epinephrine (E). DT completely suppressed the IIH-evoked elevation of NE, whereas it attenuated the E response by 20%, with the DT-induced decrease in E release failing to achieve statistical significance. DT completely blocked the release of both NE and E elicited by CPT. We conclude that specific delta-opioid receptor stimulation exerts an inhibitory effect on NE release induced by both IIH and CPT. These findings provide evidence that delta-opioid receptors may influence the autonomic sympathetic reactivity.