Neurotensin (NT) was isolated in pure form from the small intestine of the European green frog, Rana ridibunda, and its primary structure was established as pGlu-Ala-His-Ile-Ser-Lys-Ala-Arg-Arg-Pro-Tyr-Ile-Leu. This sequence contains five amino acid substitutions (Leu2-->Ala, Tyr3-->His, Glu4-->Ile, Asn5-->Ser, and Pro7-->Ala) compared with human NT. A peptide with identical chromatographic properties was identified in an extract of frog brain. Synthetic frog NT produced a concentration-dependent increase in alphaMSH release from perifused frog pars intermedia cells, with an ED50 of 5 x 10(-9) M. A maximum response (276.3 +/- 45.5% above basal release) was produced by a 10(-8) M concentration. Repeated administration of NT to melanotrope cells revealed the occurrence of a rapid and pronounced desensitization mechanism. The data are consistent with a possible role for the peptide as a hypophysiotropic factor in amphibians.