The latter confers biologic specificity to the two ligands and determines the levels of functional FSH and LH. LH stimulates testosterone production by Leydig cells whereas FSH stimulates the Sertoli cells to regulate spermatogenesis by secreting various factors that will affect Leydig cell function. The exposure to many environmental toxicants leads directly to a remarkable decline in spermatogenesis function and fertility of animals and humans. Several recent studies have demonstrated the reproductive toxicity of MCs to mammals. It has been reported that exposure to MC-LR affects male reproductive organs and reduction of sperm number in mice. In the present study, our results indicated that spermatogenesis was significantly inhibited in mice treated with MC-LR compared with the vehicle-treated mice, and MC-LR exposure significantly decreased serum testosterone and LH level while FSH concentrations kept unchanged. As the secretion of testosterone is regulated by LH, the decreased serum testosterone level was as a result of low LH levels in mice. Although intratesticular testosterone levels were shown to be significantly higher than serum testosterone levels, the intratesticular testosterone highly correlated with serum testosterone. Thus, the decreased serum testosterone level indicated the inhibited testosterone production in the testis which lead to impaired spermatogenesis. Recent studies SCH772984 showed that MCs accumulated in testis, and exerted toxic effects on reproductive system. However, the in vitro experiments in the present study demonstrated that exposure to MC-LR had no effect on the viabilities and testosterone levels of Leydig cells which locates in the testicular interstitium and are the primary cells that synthesize and secrete testosterone in adult male animals. Thus, our data suggested that MC-LR did not affect testosterone synthesis by directly damaging Leydig cells. The biological action of LH and FSH in gonadal tissue is mediated via membrane receptors for LH and FSH, respectively. In the male animals, LHR is maily expressed in testicular. Leydig cells and on ligand binding, it stimulates androgen production. FSH binds to receptors FSHR on the surface of Sertoli cells and functions in concert with testosterone to promote the spermatogenesis function. In the present study, there was no effect of MC-LR i.p. injection on the plasma level of FSH in mice, which was later confirmed by the RT-PCR results which showed MC-LR did not affect the mRNA expression of FSHb in the pituitary and FSHR in the testis, indicating that there was a normal interaction between FSHR and FSH in the testis of MC-LR-treated mouse, and MC-LR was not able to inhibit the FSH synthesis and secretion. However, the levels of LHb transcripts in MC-LR-treated mice were suppressed with decreasing the LH synthesis in the pituitary, whereas the LHR expression was not affected, suggesting that Leydig cells in testis received the impaired LH pulse via LHR, which lead to inhibited.