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In order to clarify further the role of FSH in the maintenance of spermatogenesis, adult rats were treated with purified human FSH (2 X 5 IU/day per rat), testosterone (1.5 cm silicone elastomer implant) or a combination of both hormones for 2 weeks following hypophysectomy. After hypophysectomy alone, no elongate spermatids were observed and the numbers of pachytene spermatocytes and round spermatids observed were reduced when compared with untreated controls. Testosterone supplementation alone qualitatively maintained the formation of elongate spermatids in most seminiferous tubules, whilst in FSH-treated rats increased numbers of round spermatids and pachytene spermatocytes were observed when compared with hypophysectomized animals. Formation of elongate spermatids, however, did not occur under FSH treatment alone. A combination of FSH and testosterone treatment maintained spermatogenesis in an almost quantitative fashion. Numbers of pachytene spermatocytes and round spermatids were maintained at about 80% of levels seen in intact control animals. Treatment with FSH or testosterone alone maintained testis weights at significantly higher levels than those seen in hypophysectomized controls (FSH, 0.79 +/- 0.05 g; testosterone, 0.81 +/- 0.07 g; hypophysectomized, 0.50 +/- 0.04 g). Animals treated with FSH and testosterone showed testis weights 20% below control values (1.22 +/- 0.05 vs 1.51 +/- 0.06 g; P less than 0.05). No increases in intratesticular or intratubular androgen concentrations or in testosterone: dihydrotestosterone ratios were observed in any of the hormone-treated groups when compared with hypophysectomized controls. In all hypophysectomized animals testicular androgen concentrations were reduced to less than 5% of control values. The results obtained in this study suggest that FSH is involved in the maintenance of spermatogenesis in the adult rat and that the effects of FSH are not mediated through changes in intratesticular androgens. Low levels of testosterone in combination with FSH can almost quantitatively maintain spermatogenesis in adult rats.