SUMMARY The possibility of genetic variation in the amino acid composition of neurohypophysial hormones of the mouse was investigated. Acetic acid extracts from acetone-dried neurohypophyses from six strains of mice were subjected to thin-layer chromatography. Extracts from two strains were tested for natriferic potency on a toad bladder preparation. Extracts from three strains of mice were purified on a carboxymethylcellulose column, and the amino acid composition determined by paper electrophoresis at pH 2. All six strains of mice were found to elaborate an oxytocic principle with the chromatographic properties of oxytocin. All results were compatible with the view that five of the strains elaborate the usual mammalian vasopressin, [8-arginine]-vasopressin. However, chromatography, natriferic assays and analyses of amino acid composition indicated that the Peru strain of mice elaborated [8-lysine]-vasopressin. These mice would be the first mammals outside the Suina which possess this hormone, and the difference between the strains of mice is almost certainly genetic in origin. The significance of genetic variation within a species in the structure of a hormone is discussed in terms of its physiological and evolutionary consequences.