A number of testicular peptides appear to have endocrine and/or paracrine functions. Most, if not all, of these peptides are secreted by the Sertoli cells into the tubular fluid. Although their physiological role has not been fully clarified, these secretory products are believed to play an important role in the local regulation of testicular functions, particularly the spermatogenic process. Presence of high affinity, specific binding sites for transferrin and androgen binding protein, both secreted by the Sertoli cells, has been demonstrated on the round spermatids and/or pachytene spermatocytes. Since within the seminiferous tubules, only the Sertoli cells respond directly to follicle stimulating hormone and testosterone, they could mediate the hormonal effects on spermatogenesis by providing certain products which may be essential during specific stages of germ cell differentiation.