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Here we report the solution structure of glycine-extended gastrin 17, determined by NMR spectroscopy. The spectral changes observed upon the addition of ferric ions revealed that Glu 7 acted as a ligand at the first ferric binding site, and that Glu 8 and Glu 9 acted as ligands at the second ferric ion binding site. Fluorescence quenching experiments confirmed that a GglyE7A mutant bound only one ferric ion. The inability of this mutant to stimulate proliferation or migration in the IMGE-5 cell line and the observation that the iron chelator desferrioxamine selectively blocked the effects of glycine-extended gastrin 17 indicated that binding of a ferric ion to Glu 7 was essential for biological activity. This is the first report of an essential role for a metal ion in the action of a hormone.