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SUMMARY Testosterone and androstenedione were determined chemically in testes of bulls ranging in age from 28 days to 17½ years. The earliest age at which it was possible to detect both steroids was 39 days. Their combined contents in the testes varied from 4·7 to 61 μg at 39–90 days, from 31 to 475 μg at 3½–5½ months, and from 155 to 3450 μg at 10½ months-17½ years of age. The ratio androstenedione/testosterone decreased with age. In immature bull calves, less than 4 months old, it exceeded 1:1, in animals above 9 months of age it was less than 1:10. The appearance of chemically demonstrable amounts of testosterone and androstenedione in the testes of young bull calves coincided with the onset of an active process of fructose and citric-acid production by the seminal vesicles. Between the 2nd and 6th month of life, the weight, fructose level and citric-acid level of the seminal vesicles increased at an exponential rate, and this sharp increase in the activity of the seminal vesicles was accompanied by a similarly sharp increase in the testosterone content of the testes. In adult bulls, differing widely in age and breed, the testicular content of testosterone was significantly correlated with the weight, fructose content and citric-acid content of the seminal vesicles.

It is a well-established fact that the weight, size, histological appearance and secretory functions of certain accessory organs of reproduction, such as the seminal vesicle and the prostate, are strictly dependent on, and closely regulated by, the internal secretion of the testes; furthermore, that the typical retrogressive changes which occur in the accessory glands after removal of testes can be counteracted by injections of the testicular hormone, thus serving as 'indicator tests' for the male sex hormone

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