Testicular development was studied in Tupaia belangeri (tree-shrew) from birth to sexual maturity. At birth the seminiferous cords contained peripheral supporting cells and centrally located gonocytes. Large foetal Leydig cells were prominent in the interstitium. The mitotic index of the gonocytes was low at birth and rose to peak levels at Day 20, following the regression of the foetal generation of Leydig cells, and during the nadir in circulating testosterone concentrations. Mitotic activity returned to low levels at Day 30 in association with the reappearance of differentiated Leydig cells and the first signs of increased androgenesis. The negative temporal relationship between mitogenesis and androgenic function suggests that the proliferation of the gonocytes does not require, and may be inhibited by, high titres of androgens. Post-mitotic development of the gonocytes occurred during a period of rising testosterone levels, and the first appearance of spermatogonia coincided with peak testosterone levels. This indicates that androgens may be specifically involved in the initiation of spermatogenesis. Spermatogenesis progressed to completion during a phase of declining testosterone levels. The precise temporal correlations established during post-natal development suggest that the tree-shrew is a suitable animal model for studies on the endocrine control of the initiation of spermatogenesis in primates.