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One of the major proteins regulated by FSH and secreted by Sertoli cells into the lumen is ABP. In the present study, LM immunocytochemistry revealed that ABP was expressed intensely in Sertoli cells, but the reaction was dramatically reduced in FORKO mice, a finding confirmed by the Western blot analyses. This may be due to the dilated nature of the Sertoli cell cytoplasm and its effect on secretory organelles, such as the Er and Golgi apparatus, which appeared to be less prominent than in wild-type mice and which would affect secretion. However, it is equally possible that regulation is affected at the mRNA level. As ABP is a major carrier protein delivering high concentrations of testosterone to the epididymis, it is also likely that the epididymis, an androgen-dependent tissue, could be compromised in FORKO mice. Altered epididymal functions could result in many sperm aberrations that we have previously described in FORKO mice accounting for their reduction in fertility. We are presently investigating the epididymis with respect to its structure and functions of its epithelial cells and sperm motility parameters in FORKO mice. Thus, aside from the apparent effect on water balance in Sertoli cells, FSH-R signaling also appears to be a main player in regulating ABP production, which may have an indirect effect on epididymal functions and sperm motility.