Endothelin-1 is an endothelium-derived vasoconstrictor peptide whose plasma levels are increased in patients with colorectal cancer, and which may be involved in tumor blood flow regulation. To study whether response to this peptide is altered in tumor arteries, mesenteric arteries supplying blood flow to colorectal tumors, and mesenteric arteries far from said tumors were obtained from 13 patients undergoing colectomy; mesenteric arteries were also obtained from patients with diverticulitis (n = 4) or inflammatory bowel disease (n = 3). Arteries were prepared for isometric tension recording in an organ bath, and in this preparation it was found that endothelin-1 induced contraction in all three types of arteries, but that sensitivity to this peptide was greater in arteries supplying blood flow to the tumor than in arteries far from the tumor or arteries from patients without cancer. These results suggest that endothelin-1 may regulate blood flow to colorectal tumors by inducing a greater contraction in tumor-supplying arteries than in non-tumor arteries.