The effects of cyproterone acetate (CPA), an antiandrogenic steroid, on the ultrastructure of the epithelial cells of the human epididymis were studied. Alterations in the cell size, in the cytoplasmic and surface characteristics, and in the morphology of different organelles are described. The different ultrastructural changes observed suggest that the absorptive and secretory functions of the principal cells are impaired following the CPA treatment. By contrast, with the drastic response of the principal cells of the ductuli efferentes and caput epididymidis, the epithelial cells of the cauda epididymidis appeared less affected. The suggestion of a differential androgen dependence among the different regions of this androgen target organ, as well as between the major cell-types of the epididymal epithelium, is briefly discussed.