Abstract. This study reveals that cultivation of preantral and early antral (< 500 μm) follicles in culture medium containing FCS results in an expression of cytochrome P450 _arom. (aromatase). The enzymatic activity of aromatase, measured in terms of the estradiol synthesis, was proved to be present in follicles greater than 100 μm, could further be stimulated by FSH in follicles greater than 300 μm diameter. The enzyme protein and the gene expression were studied by means of western blot and immunohistochemistry as well as by means of realtime PCR. Neither the protein nor the corresponding mRNA could be found in uncultivated follicles and in FCS-free cultivated follicles. Estradiol synthesis could not be determined under FCS-free conditions. The expression of the FCS effect was dependent on the follicle size.