The role of FSH in spermatogenesis was investigated in nonhuman primates depleted of testosterone by GnRH antagonist treatment. The GnRH antagonist antide (Nal-Lys; [N-acetyl-D-2-naphthyl-Ala1,D-4-chloro-Phe2,D-pyridyl-Ala3, nicotinyl-Lys5,D-nicotinyl-Lys6,isopropyl-Lys8,D-Ala10 ]-GnRH) was used at a daily dose of 450 micrograms/kg to suppress endogeneous gonadotropin and androgen production. Four groups of five cynomolgus monkeys (Macaca fascicularis) were subjected to the following treatment throughout a 16-week period: vehicle (group 1), GnRH antagonist (group 2), and GnRH antagonist plus human FSH (Fertinorm; 2 x 15 IU/day.animal; hFSH) during weeks 0-8 (group 3) or 8-16 (group 4). Testicular biopsies were performed before and after 4, 8, and 16 weeks of treatment. The tissue was analyzed by light microscopy and flow cytometry. Serum testosterone levels were suppressed into the range of orchidectomized animals in all GnRH antagonist-treated groups. In the absence of hFSH, serum inhibin levels were also markedly lowered. Concomitant administration of hFSH attenuated the GnRH antagonist-induced reduction of testicular size, while delayed treatment with hFSH failed to restimulate testicular volume. Numbers of A-dark spermatogonia, the reserve stem cells, were not altered by any of the treatments. hFSH either fully maintained or increased the counts for A-pale spermatogonia (renewing stem cells). The development of pachytene spermatocytes and round and elongated spermatids was markedly reduced or inhibited by the GnRH antagonist within 6-18 weeks. In contrasts, hFSH maintained these cell types at about 50% of baseline for 8 weeks. After 8 weeks of GnRH antagonist administration, hFSH stimulated A-pale spermatogonia and spermatocytes 2- to 3-fold with only minor effects on spermatid numbers. By means of flow cytometry, testicular cells were quantified according to DNA content. Within 8-16 weeks of GnRH antagonist treatment the percentage of 4C (mainly primary spermatocytes), 1C (round spermatids), and 1CC cells (elongated spermatids) had fallen from 65-75% to 5-25%. hFSH completely maintained the relative number of these cells, but failed to significantly restimulate the formation of 1CC cells.(ABSTRACT TRUNCATED AT 400 WORDS)