It has not previously been suggested that prolactin may be involved in mental illness. Yet virtually every drug used in psychiatry either stimulates or suppresses prolactin secretion (Fluckiger, 1972). Prolactin causes renal sodium, potassium and water retention (Horrobin, 1973). It is bound to cerebral tissue and alters hypothalamic activity (Turkington and Frantz, 1972; Clemens, Gallo, Whitmoyer and Sawyer, 1971). Prolactin levels up to 100 ng./ml. potentiate the responses of smooth muscle cells to noradrenaline, while higher levels inhibit the responses: preliminary studies suggest that nerve cells behave in a similar way (Manku, Nassar and Horrobin, 1973). Emotional stress, surgery and drugs such as phenothiazines, reserpine and methyldopa can elevate human plasma prolactin levels into the 50–200 ng./ml. range (Frantz, Kleinberg and Noel, 1972; Friesen, Belanger, Guyda and Hwang, 1972).