SUMMARY Three neurophysins have been isolated from acetone-dried sheep posterior lobe powder and have been named neurophysins I, II and III. Neurophysins I, II and III account for approximately 15, 15 and 70%, respectively of the total neurohypophysial hormone-binding proteins present in the sheep posterior pituitary lobe. All the neurophysins bind oxytocin and [8-arginine]-vasopressin. Neurophysins I and II each were found to contain one molecule of histidine/molecule of protein but to lack methionine, whereas neurophysin III contained no histidine but had one molecule of methionine/molecule of protein. The molecular weights of sheep neurophysins I, II and III were 11348, 9803 and 10377, respectively, as determined by amino acid analysis. An approximate value of 10000 was estimated for the apparent molecular weight of the neurophysins by gel exclusion chromatography on Sephadex G-75. Solid amorphous complexes were formed between the neurophysins and oxytocin as well as with [8-arginine]-vasopressin. In each case one molecule of neurophysin complexed with one molecule of hormone. The stoichiometry of the sheep neurophysins relative to the neurohypophysial hormones is discussed.