In most mammalian species the aromatase is encoded by a single gene (cyp19), which contains 18 exons, 9 of them being translated. In adult rats, together with Leydig cells germ cells represent an additional source of estrogens. The amount of P450arom transcript is threefold higher in pachytene spermatocytes compared to younger cells (spermatogonia-preleptotene spermatocyte) or round spermatids; conversely, aromatase activity is more intense in haploid cells. In man besides Leydig cells, we have shown the presence of a biologically active aromatase and of estrogen receptors (ER alpha and ER beta) in immature germ cells and ejaculated spermatozoa. Concerning aromatase, a 30% decrease of the amount of mRNA is observed in immotile compared to motile sperm fraction from the same sample; moreover, the aromatase activity is diminished. We have amplified aromatase mRNA by RT-real time PCR in spermatozoa from asthenospermic, teratospermic, and asthenoteratospermic men and recorded respectively 44, 52, and 67% decreases of the amount of transcripts as compared to controls. Statistical analyses between the sperm morphology and the aromatase/GAPDH ratio have revealed a high degree of correlation (r = -0.64) with the percentage of abnormal spermatozoa (especially microcephaly and acrosome malformations). Alterations of sperm number and motility have been described in men genetically deficient in aromatase, which together with our data, suggest a likely role for aromatase/estrogens in the acquisition of sperm motility. Therefore besides gonadotrophins and testosterone, estrogens produced locally should be considered as a physiologically relevant hormone involved in the regulation of mammalian spermatogenesis.