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The neurohypophysial hormones of three species of birds belonging to three different families : the chicken Gallus gallus (Phasianidae), the goose Anser anser (Anatidae) and the turkey Meleagris gallopavo (Meleagridae) have been isolated and characterized by amino acid composition, chromatographic and electrophoretic migrations and pharmacological properties. Mesotocin ( [Iles]oxytocin) and arginine vasotocin ([Args]oxytocin) have been identified in each case. Birds seem therefore to have the same neurohypophysial hormones as amphibians and reptiles but to differ on the one hand from fishes which have not mesotocin except the lungfishes akin to amphibians, and on the other hand from mammals which have oxytocin and arginine vasopressin or lysine vasopressin. The neurohypophysial hormones have displayed a peculiar stability during the evolution of the tetrapodes since three successive classes, Amphibia, Reptilia and Aves, have the same active principles. A double shift mesotocin-oxytocin and arginine vasotocin-arginine vasopressin has occurred between mammalian reptiles or primitive mammals and modern mammals. Each change involves a single amino acid substitution, in position 8 for the &st and 3 for the second. I n each case the substitution occurs between two hydrophobic residues : leucine for isoleucine in the transition mesotocin-oxytocin and phenylalanine for isoleucine in the transition arginine vasotocin-arginine vasopressin. There is a contrast between the relative similarity of the known biological properties of mesotocin and oxytocin and the great stability of each hormone in a given zoological group, stability which seems hardly explained by selection pressure.