In order to clarify the mechanism of synergism between follicle-stimulatory hormone(FSH) and testosterone (T) in the hormonal regulation of spermatogenesis, we studied the effects of recombinant human FSH (rhFSH) on the induction of spermatogenesis by testosterone in congenitally gonadotropin-deficient hpg mice. Weanling (day 21) homozygous hpg mice were administered daily subcutaneous injections of 1, 5, or 10 IU of rhFSH alone or in combination with a subdermal testosterone implant until day 70 of age. Spermatogenesis was quantitated by stereological estimation of germ cell populations and Sertoli cells as well as measurement of diameter of seminiferous tubules and their lumina in perfusion-fixed testes and by counting homogenization-resistant condensed testicular spermatids. Recombinant human FSH alone increased the absolute numbers of spermatogonia (x3.5-fold) and spermatocytes (x3-fold) compared with untreated hpg mice but did not significantly increase Sertoli cell numbers or form any condensed spermatids or tubular lumen. Relative to Sertoli cell numbers, rhFSH alone increased populations of spermatogonia (x2-fold) and spermatocytes (x2-fold). The addition of T to rhFSH further increased the absolute numbers of spermatogonia (x5-fold) and spermatocytes (x3.5-fold) compared with untreated hpg mice as well as increasing tubular diameter and forming tubular lumina. In addition administration of T allowed the completion of spermatogenesis in the presence of intratesticular T levels that were approximately 2% of non-hpg controls. All effects of the FSH + T combination were however no greater than the effects of the equivalent dose of T alone. The present study therefore indicates that rhFSH alone increases proliferation of premeiotic spermatogenic cells but has no effect on the completion of spermiogenesis or on Sertoli cell maturation. Furthermore we were unable to identify any additive effects of FSH with T in the hormonal regulation of spermatogenesis in the hpg mouse. This suggests that FSH or T both may stimulate initial spermatogenic development, but only T can complete spermiogenesis.