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-17β 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.