Peroxisome proliferator-activated receptor a (PPARalpha), a member of the steroid hormone receptor superfamily, has been linked to lipid homeostasis and tumorigenesis in tissues with high expression of receptor protein. On the other hand, the role of PPARalpha in tissues with a lower expression is not well known. Here we demonstrate the localization of PPARalpha messenger RNA (mRNA) and protein in developing and adult rat testis. Additionally, we demonstrate the expression of PPARalpha protein in adult human testis. Our experiments with Northern analysis, in situ hybridization and immunocytochemistry reveal a complex distribution of PPARalpha in tubular and interstitial cells of both adult and developing rat testis. The overall expression is rather low but may be modified by exogenous or endogenous stimuli. An up-regulation of PPARalpha mRNA could be observed after stimulation with FSH. In the developing rat testis, a clear expression of PPARalpha mRNA was present from the first days after birth. Additionally, PPARalpha mRNA and protein increased toward adulthood. In adult human testis PPARalpha immunoreactivity (IR) was present in interstitial Leydig cells and tubular cells. In the seminiferous epithelium of adult human testis the expression of PPARalpha-IR could be seen in meiotic spermatocytes, spermatids and myoid peritubular cells. The findings of our study suggest that PPARalpha may be involved in the regulation of growth and differentiation of tubular and interstitial cells in rat and human testis.