Recently it has been found that testosterone can maintain and restimulate serum and pituitary follicle-stimulating hormone (FSH) in the gonadotropin-releasing hormone (GnRH) antagonist treated adult male rat. The present investigation was undertaken to determine (1) which metabolite of testosterone, dihydrotestosterone (DHT), or estradiol accounts for the effects of testosterone in GnRH antagonist suppressed rats and (2) whether these effects of testosterone are influenced by other testicular factors. Eight groups of 6–8 adult male Sprague-Dawley rats were subjected to the following treatments: vehicle, GnRH antagonist (75 µg/day s.c), testosterone-filled Silastic implants (3 × 5 cm, s.c), DHT-filled Silastic implants (3 × 5 cm, s.c), estradiol ben-zoate (15 µg/day s.c), and combined administration of GnRH antagonist with either steroid. In addition, the GnRH antagonist/ testosterone treatment regimen was applied to rats orchidectomized 72 h prior to initiation of treatments. After 3 weeks of treatment, serum was analyzed for concentrations of luteinizing-hormone (LH), FSH, testosterone, DHT, and estradiol. Pituitary extracts were analyzed for LH and FSH content. Except for the vehicle-treated groups, serum and pituitary LH concentrations were markedly suppressed by all treatments. In intact rats treated with GnRH antagonist alone and/or estradiol, the pituitary FSH level was reduced by more than 70% relative to controls, while both testosterone and DHT maintained pituitary FSH. Similarly, testosterone and DHT, but not estradiol, delayed the decline of serum FSH induced with GnRH antagonist alone. In orchidectomized animals, testosterone was also capable of preventing a reduction of pituitary FSH despite concomitant GnRH antagonist administration. It is concluded that testosterone and DHT selectively maintain FSH in the GnRH antagonist treated male rat. This paradoxical stimulation of FSH occurs in the absence of other testicular factors.