The polypeptide hormones of the posterior pituitary gland, oxytocin and vasopressin, can be precipitated out of solution by the addition of sodium chloride to an extract of the gland. The hormones are salted-out complexed to the soluble proteins known as neurophysins and this protein-hormone complex has been found to possess the pressor and oxytocic activities in ratios similar to those found in the fresh gland (van Dyke, Chow, Greep & Rothen 1942). Neurophysins have been isolated from the pig (Uttenthal & Hope, 1970), ox (Rauch, Hollenberg & Hope, 1969) and cod (Pickering, 1968). In the present paper the proteins precipitated from the posterior lobe of the human pituitary gland were investigated by gel exclusion chromatography and starch gel electrophoresis, and their ability to bind oxytocin and vasopressin was determined. Acetone-dried posterior pituitary lobes (2·4 g) were extracted for 24 h at 4 °C in 0·1 m-hydrochloric acid (100 ml),