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 experi¬ ments the relative efficiency of these two types of gonadotrophic extract have been compared in their ability to maintain and restore the repro¬ ductive system of hypophysectomized male rats. MethodsMale albino rats of the Wistar Institute strain, 2| 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 the sella was sectioned for histological study. Cases of incomplete hypophysectomy are not included in this series.In one group of rats treatment was commenced immediately after hypophysectomy, and in the second and third series 2 weeks and 4 weeks respectively were allowed to elapse before treatment was started. The treatment was continued for 4 to 6 weeks. In another series no treatment was given, but autopsies were performed at various intervals after opera¬ tion to ascertain the rate of atrophy of the reproductive organs.The pregnant mare serum extract was Antex Leo, assayed on immature mice, one unit being described as the amount of the extract which doubles the weight of the ovaries. The pregnancy urine preparation was Physex

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