Hylambatin is the first example of a tachykinin which possesses a methionyl methionine residue at the C-terminus, rather than the C-terminal tripeptide -Gly-Leu-Met-NH2 which hitherto has been a characteristic feature of all members of the tachykinin family. The effect of hylambatin on the secretion of glucoregulatory hormones was examined in the rat. Hylambatin, injected intravenously in graded doses 10 and 30 min before blood collection, significantly increased both plasma glucose and plasma insulin, whereas the secretion of glucagon was not affected. This profile of action is different from that of kassinin or substance P. Should hylambatin, like other neuropeptides, be present in mammalian tissue, it may have a role in the regulation of carbohydrate metabolism.