We investigated the effects of GnRH analogs, different doses of testosterone (T), an androgen receptor antagonist (flutamide), and combinations of these on the recovery of spermatogenesis after irradiation. Treatment with a GnRH agonist (Lupron) for 10 weeks after irradiation reduced the intratesticular T concentration (ITT) to 4% of that in irradiated rats and serum FSH to undetectable levels without altering serum LH levels. Injection of a GnRH antagonist (Cetrorelix) at 3 weeks after irradiation suppressed LH, FSH, and ITT to <7%, 32%, and 10%, respectively, of levels in irradiated-only rats within 2 weeks; suppression was maintained for approximately 3 to 4 weeks. The percentage of tubules with differentiated germ cells (repopulation index, RI) was <0.6% at weeks 10 to 20 after irradiation. Spermatogenic recovery was induced by both the GnRH agonist (RI = 58% at week 10; 91% at week 20) and antagonist (RI = 70% at week 13). There was a dose-dependent suppression of testicular germ cell repopulation when T was combined with GnRH analogs. The ability of T to abolish the spermatogenic stimulatory effect of the GnRH antagonist was evident by the similar RI obtained for irradiated rats given antagonist + T or T alone. This suppression of GnRH-induced recovery of spermatogenesis by T could be reversed by flutamide. The RI best correlated with the degree of ITT suppression. In ITT-suppressed rats, the RI also showed an inverse correlation with serum T levels. Thus, T and/or its androgenic metabolites either directly or indirectly inhibit spermatogenic recovery after irradiation through an androgen receptor-mediated process. In addition, there was a close negative correlation between RI and FSH levels, and hence, a spermatogenic inhibitory role for FSH in the irradiated rats cannot be ruled out.