Abstract: Objective: To observe the effects of intravenous and intracerebroventricular injections of endomorphin-1 (EM-1) on serum nitric oxide (NO) content and nitric oxide synthase (NOS) activity of rats.Methods: Twenty Wistar rats were randomly divided into two groups:intravenous injection group (n=14) and intracerebroventricular injection group (n=6).The right femoralartery was cannulated with polyethylene tubing to record arterial pressure.Variations of arterial pressure, NO content and NOS activity were observed after intravenous or intracerebroventricular injection of EM-1 in rats.The effects of intravenous injection of various blockers cyprodime, N-ω-nitrolarginine, on the varitation MAP, NO content and NOS activity caused by EM-1 were observed.Results: Intravenous injection of EM-1 produce depressor on mean arterial pressure (P<0.01) in anesthetized rats.NO content and NOS activity in serum were much higher than those in control group (P<0.01).While intravenous injection L-NNA and cyprodime could partially antagonize the effects;intracerebroventricular injection of EM-1 produced depressor on mean arterial pressure (P<0.01) in anesthetized rats.NO content and NOS activity in serum were no more than those in control group (P>0.05).Conclusions: Intravenous injection of EM-1 produces depressor on mean arterial pressure may be most associated with the release of nitric oxide from vascular endothelium.While intracerebroventricular injection of EM-1 produces depressor on MAP may be most associated with central opioid receptor.