Aquaporins (AQPs) are membrane protein channels that allow the rapid passage of water through an epithelium containing tight junctions. In the present study, light microscope immunocytochemistry was utilized to localize several members of the AQP family in the testis, efferent ducts, and epididymis of normal adult animals during postnatal development and after various experimental procedures on adult animals. In the testis of normal adult animals, AQP-8 was expressed exclusively in Sertoli cells, while AQP-9 outlined Leydig cells. In the efferent ducts, AQP-1 was expressed on the microvilli, basolateral plasma membranes, and apical endosomes of the nonciliated cells and cilia of ciliated cells, while AQP-9 was present only on the microvilli of nonciliated cells. In the epididymis, AQP-9 was localized to the microvilli of the principal cells of all regions, with the most intense reaction being noted in the initial segment and cauda regions. The clear cells of the cauda region expressed only AQP-9. AQP-1 was not expressed in the testis or the epididymal epithelium, but it was expressed over the endothelial cells of the vascular channels of the efferent ducts and epididymis. After efferent duct ligation or orchidectomy, there was no change in the expression of AQP-1 or -9 over the microvilli or cilia of epithelial cells in the case of the efferent ducts, suggesting that testicular factors do not regulate their expression in this region. In contrast, AQP-9 expression in the principal cells of the initial segment, but not of other regions, and also in the clear cells of the cauda region was dramatically reduced after both treatments. As the expression was not restored to control levels by testosterone replacement, the data suggest that a luminal factor(s) derived from the testis regulates AQP-9 expression in the principal cells of the initial segment and in the clear cells of the cauda region. Postnatal studies revealed that the expression of AQP-1 and -9 in the different cell types of the efferent ducts and epididymis occurred between days 7 and 29, eliminating sperm and high androgen levels as possible regulating factors. Taken together, these data suggest cell specificity with respect to the expression of AQP-8 and -9 in the testis. In the efferent ducts and epididymis, specificity exists in cell, region, and tissue distribution with respect to the expression of AQP-1 and -9, and their expression does not appear to be regulated by androgens.