This study describes the synthesis and effects of the first antagonist to the widely distributed neuropeptide, galanin, which inhibits the secretion of insulin. The first galanin antagonist is a 20-amino acid-long chimeric peptide of the composition galanin-(1-12)-Pro-substance P-(5-11) amide: Gly-Trp-Thr-Leu-Asn-Ser-Ala-Gly-Tyr-Leu-Leu-Gly-Pro-Gln-Gln-Phe-Phe-Gly- Leu-Met amide. The peptide dose dependently (IC50 = 1.0 nM) antagonizes the galanin-mediated inhibition of the glucose-induced insulin secretion from mouse pancreatic islets. The antagonist was also found to displace 125I-monoiodo-[Tyr26]galanin from membranes of the insulin producing Rin m 5F cells with an IC50 value of less than 0.1 nM. The antagonist is named galantide.