In order to define the respective involvement of steroidogenesis enzymes subtypes in the control of hair follicle homeostasis, we evaluated, by semiquantitative RT/PCR, the expression levels of mRNAs coding for 17 β-hydroxysteroid dehydrogenase type 1 and type 2, 3β-hydroxysteroid dehydrogenase, Cyt.P450-aromatase, steroid 5α-reductase type 1 and type 2 and 11 β-hydroxysteroid dehydrogenase. These assays were performed for several components of the pilosebaceous unit (PSU); fresh plucked anagen hairs, sebaceous glands and primary culture of dermal papilla, as well as other tissues involved in an active steroid metabolism (human testis, liver, placenta, prostate, ovary, uterus and adrenals) as controls. We found that plucked hair (i.e. mainly keratinocytes from the inner and outer root sheaths) expressed: (1) very high levels of 17β-hydroxysteroid dehydrogenase type 2 corresponding to levels found in liver and placenta; (2) high levels of steroid 5-α-reductase type 1 corresponding to levels found in testis, liver and ovary, and moderate levels of 17 β-hydroxysteroid dehydrogenase type 1, which corresponded to the expression in testis, prostate and uterus. In contrast, Cyt.P450-aromatase, 3β-hydroxysteroid dehydrogenase and steroid 5α-reductase type 2 were poorly expressed in the pilosebaceous unit as compared with other tissues. Interestingly, expression patterns of these enzymes in primary cultures of dermal papilla were distinctive since 5α-reductase type 1 and 11 β-hydroxysteroid dehydrogenase were the only mRNA detected. Taken together, these results suggest that not only sebaceous gland but also outer root sheath keratinocytes may contribute, through the activity of the steroid 5α-reductase type 1, to the pathogenesis of androgen-depende