Oestrogenic hormones were originally isolated from ovarian follicles and from placental tissue and were believed to occur only in female animals. Laqueur et al. (1927} observed, however, that extracts from human male urine caused vaginal cornification in spayed mice. This discovery of oestrogenic activity in urine from men was so unexpected, that the authors thought it necessary to state that there could be no doubt about the manliness of the subjects studied. One of the oestrogenic substances in human male urine was subsequently identified as oestrone (Dingemanse et al., 1938), while later on oestradiol and oestriol were also found to be present in urine from men (see: Diczfalusy & Lauritzen, 1961). Since then, the occurrence of oestrogens in the urine of male animals from several species has been described (see: Velle, 1966). However, information on the precise origin of these oestrogenic hormones and on the regulation of the production of oestrogens in the male animal is still limited. Therefore, it was decided to investigate these points with special reference to the testis as a possible source of oestradiol.