Estrogens are key regulators of sexual differentiation and development in vertebrates. The P450 aromatase (P450arom) is the steroidogenic enzyme responsible for the synthesis of estrogens from androgens. In the adult rat testis, aromatase transcripts and activity have been observed in somatic cells and germ cells, including pachytene spermatocytes (PS) and round spermatids (RS), but little is known concerning regulation of the aromatase gene expression, especially in germ cells. The quality of germ cell preparations was assessed by the absence of androgen-binding protein and stem cell factor transcripts, two specific markers for Sertoli cells. By employing a competitive quantitative reverse transcriptase-polymerase chain reaction technique, we confirmed that germ cells contained P450arom transcripts and demonstrated that the aromatase gene was up-regulated by cAMP. Conversely, transforming growth factor (TGF) β1 inhibited Cyp19 gene expression in a dose- and a time-dependant manner in both PS and RS. The addition of tumor necrosis factor (TNF) α to purified germ cells induced an increase of the amount of P450arom mRNA in PS, although an inhibitory effect was observed in RS. When PS were treated with dexamethasone (Dex), a similar enhancement of the aromatase transcript level was observed, whereas an inhibitory effect was recorded for RS. Furthermore, in either TGFβ1- or TNFα-treated germ cells, the addition of Dex stimulated the aromatase gene transcription. Experiments using 5′ rapid amplification of cDNA ends suggested that promoter PII is mainly concerned in the regulation of the aromatase gene expression in germ cells of adult male rats; however, the presence of other promoters could not be excluded.