SUMMARY: Testosterone, androstenedione and 17α-hydroxyprogesterone were identified as secretory products of the bull testis by concurrent analyses of the arterial and spermatic vein blood. The testes of prepuberal calves aged 3–6 months (86–200 kg. body weight) were shown to secrete testosterone at rates varying from 14 to 231 μg.hr./testis. In calves of less than 125 kg. body weight, the testicular venous blood contained androstenedione in amounts exceeding those of testosterone, whereas in calves above 175 kg. in body weight and in mature bulls the androstenedione/testosterone ratio was well below 1:10. Administration of human chorionic gonadotrophin (HCG; 2000 i.u./48 hr., intramuscularly, for 7 weeks) did not abolish the high androstenedione/testosterone ratio which characterizes the secretion of the immature bull testis. Intravenous administration of HCG to mature bulls (2–5 i.u./kg.) was followed within 20 min. by an increased output of testosterone into the spermatic vein blood. This response was attended by a rise in the testosterone content of the testis, and was therefore attributed to an increased rate of hormone synthesis, rather than to a release of preformed hormone stored in the gland. There was a significant positive correlation between testosterone content of the testis and rate of testosterone release into the spermatic vein blood. A time interval of 18·0 min. ± 6·6 s.d. (mean of 22 determinations) was sufficient for the release into the testicular effluent of an amount of testosterone equal to the entire content of the gland, and for its replacement by newly synthesized hormone. 17α-Hydroxyprogesterone constituted epiandrosterone was not detectable in the spermatic vein blood by a method sensitive to a plasma concentration of 5–10 μg.% of this steroid.