The mammalian testis is a complex organ which produces spermatozoa and synthesizes steroids. The transformation of androgens into estrogens is catalyzed by aromatase, an enzymatic complex encoded by a single copy-gene (cyp19) which contains 18 exons, 9 of them being translated. In man besides Leydig cells, we have demonstrated the existence of a biologically active aromatase in immature germ cells and in ejaculated spermatozoa. In addition the presence of estrogen receptors (ERalpha and ERss) in immature germ cells and in spermatozoa has been reported. Concerning aromatase, a 30% decrease of the amount of mRNA is observed in immotile compared to motile sperm fraction from the same sample. In asthenoteratozoospermic, teratozoospermic and asthenozoospermic patients, the aromatase gene expression is decreased respectively by 67%, 52% and 44%, when compared to normospermic controls. Statistical analyses between the sperm morphology and the aromatase/GAPDH ratio have revealed a high degree of correlation (r=-0.64) between that ratio and the percentage of abnormal spermatozoa (especially microcephaly). In men genetically deficient in aromatase diminutions of sperm number and motility have been published. Therefore besides gonadotrophins and testosterone, estrogens are likely playing a relevant role in spermiogenesis and human male gamete maturation.