In our survey for unknown neuropeptides in porcine spinal cord, we have purified a novel decapeptide that exhibits a potent stimulant effect on the smooth muscle preparation of rat uterus. By microsequencing and synthesis, the peptide has been identified as Gly-Asn-Leu-Trp-Ala-Thr-Gly-His-Phe-Met-NH2. The peptide is found to have a surprising sequence homology to amphibian bombesins, and to display a potent contractile activity of rat uterus in the characteristic manner of the known bombesins. These facts strongly suggest that the peptide may be involved in the neural communication system of mammals as a neuromediator or hormone. Thus, we propose the name "neuromedin B" for the peptide.