This study was designed to investigate the function of estrogen in related with the effects of the antiestrogen on morphologic changes of efferent ductules of the adult male mouse. Estrogen is synthesized in the male reproductive system and is found in high concentrations in rete and seminal fluids. Adult male mice, 30 days old, were injected subcutaneously with ICI(5mg/mouse) once per week for 8 weeks. Tissues were fixed by vascular perfusion on days 8, 10, 12, 14, 16, 25 and 59 post-treatment. The lumen of efferent ductules began to dilate from the day 8 and continued to the day 59 post-treatment. Epithelial cell height in the efferent ductules of the treated mice decreased all the time periods, compared to the control. Cell height in proximal region of the efferent ductules in the treated groups decreased most by 38%. Supranuclear cytoplasmic height in epithelial cell of the ducts also showed the decrease of 46%, 39% and 33% in proximal, conus and common regions in the treated groups, respectively. ICI 182,780 treatment in the male mouse reproduced a similar morphological abnormalities that were observed in the efferent ductules of the ${alpha}ERKO$ mouse.