Although earlier reports suggest a stimulatory effect of FSH on Leydig cell function, controversy exists due to unavailability of FSH preparations free of contaminating LH. Recent availability of recombinant human FSH preparations made it possible to reinvestigate this question. Immature male rats were hypophysectomized (21-22 days old at surgery) and implanted with osmotic minipumps releasing 8 IU recombinant FSH or 18 IU purified human pituitary FSH (hpFSH)/day, whereas control animals received vehicle alone. After 7 days of treatment, testicular weight increased in the recombinant FSH and hpFSH-treated animals to values 2.3- and 2.5-fold those of controls, respectively. Analyses of the steroidogenic capacity of Leydig cells in testes of rats treated with recombinant FSH or hpFSH also revealed 2.9- and 3.8-fold androgen production in vitro compared to controls. In these rats recombinant FSH and hpFSH increased the LH receptor number in testicular homogenate by 50% and 70%, respectively. The increase in LH receptor number was associated with increases in the LH receptor mRNA levels. In hypophysectomized control rats, small seminiferous tubules contained spermatogonia and zygotene/early pachytene spermatocytes. In contrast, treatment with either FSH preparation enhanced the progression of meiosis, as evidenced by large number of pachytene spermatocytes and appearance of round spermatids. The present results show that LH-free recombinant FSH, like purified pituitary FSH, is capable of increasing the LH receptor content and steroidogenic responsiveness of Leydig cells through paracrine mechanisms together with a stimulatory effect on spermatogenesis. These observations suggest that prepubertal elevation of FSH secretion may be important for increasing Leydig cell steroidogenic capacity and spermatogenic progression.