The ability of the testis to irreversibly convert androgens into estrogens is related to the presence of a microsomal enzymatic complex called aromatase, which is composed of a specific glycoprotein, cytochrome P450 aromatase (P450arom) and an ubiquitous reductase. In the rat testis, we have immunolocalized P450arom not only in Leydig cells, but also in germ cells and especially in elongated spermatids. We have shown that the level of P450arom mRNA transcripts decreases according to the stage of germ cell maturation and is much higher in younger germ cells than in mature germ cells, while aromatase activity is 2- to 4-fold higher in spermatozoa compared to the other two enriched germ cell preparations. By using anin vitro model of mature rat Leydig cells, pachytene spermatocytes and round spermatids, we have shown that several factors direct the expression of the aromatase gene in these cells and that promoters PII but also PI.4 are clearly involved. Our recent data obtained from ejaculated human spermatozoa demonstrate expression of aromatase both in terms of mRNA and protein, and our results also suggest that aromatase could be involved in the acquisition of sperm motility. Together with the widespread distribution of ERs in testicular cells, these data provide new light on the hormonal regulation of spermatogenesis in mammals.