The relation between endothelin and acetylchollne (ACh) was examined and compared in aortas from Wistar-Kyoto (WKY) rats and from stroke-prone spontaneously hypertensive rats (SHRSP). The relaxation produced by ACh in an endothelin-induced contraction was less in aortas from WKY rats than in those from SHRSP. In aortas from WKY rats but not in those from SHRSP, the contraction produced by endothelin was augmented when the intact aortic rings were treated with methylene blue (10−5M). This augmentation was also found in preparations of the WKY rat aortic rings in which the endothelium had been removed. The augmentation was not present in SHRSP aortic rings that had been similarly denuded. Treatment with indomethacin (5 × 10−6M) had no effect on endothelin-induced contraction in either WKY rat or SHRSP aortic rings. Our findings indicate that endothelin and ACh have in common the ability to release endothelium-derived relaxing factor (EDRF) in WKY rat aortic rings. The reduced endothelium-dependent relaxation in response to ACh in the WKY rat probably reflects the fact that endothelin had already released the EDRF in rings from this strain of rats. The release of EDRF by endothelin is less in SHRSP than it is in WKY rats. Because of this failure of endothelin to release EDRF in SHRSP, endothelin may contribute to the increase in total peripheral resistance in this form of hypertension.