It was verified in our previous study that endothelium-dependent relaxation (EDR) in the mesenteric artery strip of adult spontaneously hypertensive rats (SHR and SHRsp) was decreased upon addition of acetylcholine (ACh). In order to explore further the mechanism of this decrease of EDR, the effects of NG-nitro-L-arginine (L-NNA) (a NO synthase inhibitor) and reduced hemoglobin (RHb) (a scavenger of EDRF) on this ACh EDR were investigated in both stroke-prone SHR (SHRsp) and control WKYs. Our results showed that the difference of EDR (ACh 10(-8)-10(-5) mol/L) between SHRsp and WKY disappeared in the presence of L-NNA (10(-3) mol/L), but addition of RHb (10(-5) mol/L) would abolish such a difference only at ACh 10(-7)-10(-8) mol/L. In comparison to changes of EDR (ACh 10(-8)-10(-5) mol/L) between post L-NNA and post RHb in WKY and in SHRsp, no significant difference was found in WKY, but significant difference was found in SHRsp. And also, it appeared that the sensitivity of intact endothelium mesenteric artery of SHRsp to RHb was similar to WKY, but that to L-NNA is lower than that of WKY. Consequently it is considered that in the attenuation of EDR in hypertensive visceral vessels, the EDRF mechanism and endothelial L-arg-NO pathway are not identical.