Abstract RES-701-1 (cyclic (Gly 1Asp 9)(GlyAsnTrpHisGlyThrAlaProAspTrp PhePheAsnTyrTyrTrp)), a peptide isolated from Streptomyces sp., has been reported to inhibit the endothelin ET B receptor. We examined the effects of this peptide on the blood vessels and the small intestine. In isolated rat aorta without endothelium, 10 μM RES-701-1 did not affect the resting tone, nor did it attenuate the contractions induced by endothelin-1, endothelin-3 or norepinephrine. In the aorta with endothelium, 3 μM RES-701-1 shifted the concentration-response curves for the contractile effects of endothelin-1 and endothelin-3 to the left. Removal of endothelium showed a similar effect to 3 μM RES-701-1. In the norepinephrine-stimulated aorta, endothelium-dependent relaxation induced by endothelin-3 was antagonized by 0.3–10 μM RES-701-1 in a concentration-dependent manner. In the guinea pig ileum stimulated by carbachol, endothelin-3 induced a transient relaxation followed by sustained relaxation. RES-101-1 (3 μM) selectively inhibited the transient relaxation. Since it has been shown that the contractile effects of endothelins in the aorta are mediated by the endothelin ET A receptor whereas the endothelium-dependent relaxation and the ileal relaxation are mediated by the endothelin ET B receptor, it is suggested that RES-701-1 is a selective antagonist against the endothelin ET B receptor.