The pituitary gland of the male adult rodent contains as much prolactin as that of immature females (Jones, Fisher, Lewis & VanderLaan, 1965), but the significance of its occurrence in the male is not clear. The effect of prolactin on male fertility was therefore studied in genetically dwarf mice (dwarf = dw and Ames dwarf = df). The adenohypophysis of dwarf mice produces little or no prolactin, even when removed from hypothalamic inhibition (Bartke, 1965b). Female dwarf mice are sterile but can reproduce normally when prolactin is provided (Bartke, 1965a, 1966). In a non-inbred stock of df dwarfs, grafting normal mouse pituitary into the renal capsule increased the proportion of fertile dwarf males. This indicates a role of prolactin in male fertility. However, pituitary grafts, in addition to secreting prolactin, have pronounced growth hormone (GH) and some thyroid-stimulating hormone (TSH) activity (Hertz, 1959) and it is known that both GH