Two molecular forms of gastrin-releasing peptide (GRP) were isolated from an extract of the intestine of the tetraploid frog Xenopus laevis. The primary structure of GRP-1 (APTSQQHTEQ¹⁰LSRSNINTRG²⁰ SHWAVGHLM.NH₂) differs from that of GRP-2 by a single amino acid substitution (Asn¹⁵ Thr¹⁵). GRP-(20-29) peptide (neuromedin C) was also isolated from the extract. Synthetic GRP-1 produced concentration-dependent contractions of longitudinal smooth muscle strips from Xenopus cardiac stomach (pD₂ = 8.93 ± 0.32; n = 6). The responses were unaffected by tetrodotoxin, atropine, and methysergide, indicating a direct action of the peptide on smooth muscle cells. GRP-1 elicited concentration-dependent relaxations of precontracted (5 µM carbachol) circular smooth muscle strips from the same region (pD₂ = 8.96 ± 0.21; n = 8). The responses were significantly (P < 0.05) attenuated (71 ± 24% decrease in maximum response; n = 6) by indomethacin, indicating mediation, at least in part, by prostanoids. Despite the fact that Xenopus GRP-1 differs from pig GRP at 15 amino acid sites, both peptides are equipotent and equally effective for both contractile and relaxant responses, demonstrating that selective evolutionary pressure has acted to conserve the functional COOH-terminal domain in the peptide. The data suggest a physiologically important role for GRP in the regulation of gastric motility in X. laevis.