Gastrins occur in the hypothalamo-hypophyseal neurons of all mammalian species examined. In addition, human, bovine, and murine hypothalamo-hypophyseal neurons contain the homologous cholecystokinins (CCKs). CCK also occurs in neurons innervating bovine melanotrophs. Although the concentration of gastrin is of the same magnitude (15-30 pmol/g) in all neural lobes, the concentration of CCK varies from undetectable in pig and cat to 1 nmol/g in the cow. The constant occurrence of neurohypophyseal gastrin suggests a role different from that of the species-dependent CCK.