The serum concentration of gonadotrophin during several stages of pregnancy was determined in seven mares bred to a stallion and in four mares bred to a jack. To eliminate individual differences, two mares were bred successively to a stallion and to a jack. When the mare is carrying a mule foetus the hormone concentration is approximately one-tenth that associated with a horse foetus. One mare carrying a mule foetus was killed on the 69th day of pregnancy. Though the endometrial cups appeared normal histologically, the endometrial cup secretion was scanty and low in potency.The amount of hormone in the cups themselves was also low.The genotype of the foetus, we conclude, influences the secretory activity of the endometrial cups.

Previous reports by other investigators on the presence of oestrone and oestradiol-17β in stallions' urine have been substantiated by chemical methods including infrared spectrophotometry. In addition a third oestrogen, oestradiol-17α, was isolated from this source. Quantitative chemical determinations of oestrogens in urine samples from 36 stallions from 7 to 37 months old, showed that only insignificant quantities were excreted during the first two years of life. At this time there was a marked increase, with oestrone predominating. In 8 mature stallions the average value for oestrone was 20.7 mg per litre of urine. Both oestradiol-17β and oestradiol-17α were excreted in amounts approximating 1/10 of this value. Using the same method only trace amounts of urinary oestrogens were recorded in 10 fully grown geldings.

Eight female sheep were treated twice weekly with various doses of equine gonadotrophin injected either subcutaneously or intravenously. Antigonadotrophic activity appeared by the second week with injections of 200 IU, by the third week with 500 IU, and by the fifth week with the highest dose (1000 IU). In the last case, maximum activity was delayed until the 7th week. Route of injection did not influence rate of formation. When injections were discontinued, the antigonadotrophic potency of the serum was reduced after three weeks, but the levels again rose rapidly upon resumption of treatment, soon equaling or exceeding the former levels. Prolonged administration of gonadotrophin did not influence the weights of the pituitary gland, adrenal, thyroid, or ovaries. When large doses of equine gonadotrophin (500 or 1000 IU) were injected twice weekly, the ovaries remained acyclic and pituitary LH content was significantly reduced.

Summary. The urinary excretion of oestrogens in mature male mules was estimated by means of the method of Brown (1955).It was found that oestrone was the major oestrogen present in the urine of normal mules. Oestradiol-17\g=b\ was also present, but in considerably smaller amounts ; oestriol could not be detected. None of these three oestrogens was detected in any of the samples of urine from castrated mules.The oestrogen excretion in the mule was compared to that in horse and the significance of the difference found was discussed.

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