Testosterone can initiate spermatogenesis in an immature nonhuman primate, Macaca fascicularis.

Four 1-yr-old, i.e. immature, monkeys (M. fascicularis) were treated with testosterone. For the first 21 weeks, they received testosterone capsules sc; thereafter for the remainder of 1 yr, they received weekly injections of 125 mg testosterone enanthate. Four similarly aged monkeys served as untreated controls. The testosterone enanthate injections produced a peak level of 345 +/- 70 nmol/liter (mean +/- SD) after 24 h and the levels were 187 +/- 39 nmol/liter 7 days later. The overall mean level of circulating testosterone in the untreated monkeys was 3.5 +/- 1.1 nmol/liter. Testicular volumes of the treated monkeys became 6 times larger than those of the untreated monkeys. The testicular testosterone concentrations of the treated monkeys were about 2- to 4-fold greater than those of untreated immature monkeys but no more than 82% of the normal adult range. Spermatogenesis in varying degrees of completeness was found in the testes of all four treated monkeys. Moreover, sperm, including some motile cells were found in the ejaculates of two monkeys. These data lead to the conclusion that spermatogenesis can be initiated, although not quantitatively, in an immature nonhuman primate by testosterone treatment.