In the mammalian testis, the irreversible conversion of androgens into estrogens is catalyzed by the cytochrome P450 aromatase, a microsomal enzymatic complex encoded by a unique gene (Cyp19) in humans which contained 18 exons, 9 of them being translated. In addition that gene includes 9 non-coding exons I, located in the 5 end and controlled by tissue-specific promoters that are spliced alternatively onto a common site in exon II. However, a unique protein of 55 kDa is produced. In most of mammals studied so far, at least in rodents, all testicular cells except peritubular cells contain aromatase which is therefore constitutively expressed and very precisely controlled (in terms of expression and enzyme activity) according to somatic and germ cell origin. In man, our data obtained from both ejaculated spermatozoa and immature gem cells demonstrate the presence of a biologically active aromatase ; moreover estrogen receptors (ER alpha and ER beta) are also present, especially ERβ in seminiferous tubules. All together according to the widespread localisation of aromatase and estrogen receptors in testicular cells, our review clearly shows that besides gonadotrophins and androgens, estrogens produced locally, should be considered as physiologically relevant hormones involved in the regulation of spermatogenesis and spermiogenesis. Therefore it is suggested that aromatase could be a helpful marker which might reflect the gene expression during spermatogenesis as well as the sperm quality and consequently, that parameter may be used as a diagnostic tool to assess the fertility in man.