To investigate the morphological changes of epithelial cells of ductus epididymides by sperm entrance, the Korean striped field mouse, Apodemus agrarius coreae was studied with light and transmission electron microscopy. 1. The diameters of the caput epididymis (Cp) and its lumen were $135sim145{mu}(140{pm}0.5{mu})$ and $115sim120{mu}m(117{pm}0.1{mu}m)$. The length and width of the epithelium were $27.0sim28.5{mu}m(28.0{pm}0.1{mu}m)$ and $4.8sim5.4{mu}m(5.1{pm}0.2{mu}m)$. 2. The diameters of the corpus epididymis (Cr) and its lumen were $160sim170{mu}m(166{pm}0.2{mu}m)$ and $140sim150{mu}m(145{pm}0.3{mu}m)$. The length and width of the epithelium were $17.4sim18.0{mu}m(17.6{pm}0.5{mu}m)$ and $8.8sim10.4{mu}m(9.5{pm}0.2{mu}m)$. 3. The diameters of the cauda epididymis (Cu) and its lumen were $270sim280{mu}m(275{pm}0.2{mu}m)$ and $265sim275{mu}m(268{pm}0.3{mu}m)$. The length and width of the epithelium were $11.2sim13.4{mu}m(12.3{pm}0.3{mu}m)$ and $9.2sim11.2{mu}m(10.0{pm}0.2{mu}m)$. Therefore, the size of diameter and lumen of the ductus epididymides and the width of the epithelium increased from Cp and Cr to Cu, but the length of epithelial cells decreased from Ca and Cr to Cu. These data suggest that the morphological changes of epithelial cells of the ductus epididymides may be the results of the sperm entrance.