The regional histology and esterase activity of the mouse epididymis after 24, 48, and 72 hr castration is reported. Differential sensitivity to androgen deprivation among the various epithelial cell types is described, allowing of positive identification of the cell types previously observed to survive long-term castration. The possibility of an androgen binding protein, as described in the rat and rabbit, is suggested on morphological grounds. The epididymal body appears to contain a class of highly androgen sensitive cells that degenerate rapidly following castration and a second class that survive from which regeneration occurs on testosterone replacement.