To determine whether testes modulate LH secretion through nonandrogenic factors, serum concentrations of FSH, LH, and testosterone were measured in adult male rats at different times after orchidectomy or treatment with ethylene dimethane sulfonate (EDS), a specific toxin for Leydig cells. The destruction of Leydig cells in EDS-treated males was confirmed by histological studies and the absence of a response to hCG stimulation. FSH and LH responses were lower in EDS-treated than in orchidectomized males despite the fact that plasma testosterone levels were undetectable, and ventral prostates were similarly atrophied in both experimental groups. When male rats treated with EDS were also injected with the antiandrogen flutamide or the antiestrogen tamoxifen, differences from castrated males remained significant. In addition, orchidectomized and EDS-treated males with similar serum concentrations of testosterone after implantation of Silastic capsules containing the androgen showed also different serum LH concentrations. These data strongly suggest that a non-Leydig factor(s) was involved in the control of LH secretion. FSH and LH responses to orchidectomy or EDS treatment were also analysed in old male rats (20-22 months) and males with damage of Sertoli cell function induced by neonatal injection with estradiol or GnRH antagonist. We found that in these experimental groups, LH secretion was similar after castration or EDS treatment. We conclude that a non-Leydig cell factor(s) controlled LH secretion and that this control mechanism does not exist in old males or males neonatally injected with estradiol or GnRH antagonist.