In this study, we determined the relative role of LH and FSH in initiating the pubertal proliferation of Sertoli cells in primates. Sixteen juvenile male rhesus monkeys (Macaca mulatta) bearing venous catheters received intermittent intravenous infusions of single chain human LH (schLH) or recombinant human FSH (rhFSH) or a combination of both for 11 days. The schLH infusion elicited a physiological testosterone response. On Day 11, monkeys were castrated, and one-half of a testis was fixed in Bouin's fluid. Infusion of the gonadotropins, either alone or in combination, effected a significant increase in testicular weight, seminiferous cord diameter, and the number of Sertoli cells per testis (schLH, 295 ± 46 × 10⁶; rhFSH, 342 ± 64 × 10⁶; LH FSH, 298 ± 26 × 10⁶ versus vehicle, 204 ± 26 × 10⁶). The latter finding indicated that LH, in addition to FSH, plays a critical role in the initiation of the pubertal proliferation of Sertoli cells in primates. Moreover, combined gonadotropin treatment led to the appearance of germ cells as mature as early primary spermatocytes, indicating that initiation of spermatogenesis had been set in motion. Because the duration of hormone stimulation was only 11 days, the latter result suggests that Leydig and Sertoli cells of the juvenile monkey testis can immediately transduce a gonadotropin signal to the germ cell.