Evans, Meyer, and Simpson [1933] reported that in male hypophysectomized rats an extract of pregnant mare serum was capable of causing regeneration of atrophied testes and resumption of spermatogenesis and fertility. The accessory organs grew to normal size. Histologically the testes of the injected animals appeared normal. Similar results have been achieved with extracts of human pregnancy urine by Smith and Leonard [1934] and Evans, Pencharz, and Simpson [1934]. In the present experiments the relative efficiency of these two types of gonadotrophic extract have been compared in their ability to maintain and restore the reproductive system of hypophysectomized male rats. Methods Male albino rats of the Wistar Institute strain, 2½ to 3 months old, weighing 150 to 190 g. were hypophysectomized by a modified Selye technique [Collip, Selye, and Thomson, 1933]. Criteria of complete hypophysectomy were body-weight curve, adrenal weight, and examination of sella turcica at autopsy. In doubtful cases