Experimental hypertension was produced by intravenous infusion of norepinephrine in 13 conscious dogs. Electroacupuncture at the dogs' "Tsu San-Li" (St. 36) points showed a significant decrease in blood pressure, while the heart rate was not affected. The depressor effect was naloxone (0.2 mg/kg iv) reversible, and accompanied by an increased blood flow at the mesenteric artery, so it is suggested that the depressor effect was due to inhibition of the sympathetic vasoconstrictor tone. This inhibition was mediated by endogenous opioid peptides released by acupuncture. The location of this mediation was further analyzed. The central mechanism was evidenced by the ineffectiveness of acupuncture in reducing blood pressure in anesthetized dogs. On the other hand, the demonstration of opiate receptors in the blood vessels by radio-receptor assay provided evidence of peripheral mediation of blood pressure by opioids. The blood vessel might be a target organ for the plasma opioids, which can also be increased by acupuncture.