Mouse epididymides were examined by light and electron microscopy 6, 12, 24h, 1.5, 2, 3, and 5 days after efferent duct cutting when the mice were 60 days of age. Six hours after operation, the principal cells in the initial segment of the epididymal duct began to degenerate following the disappearance of intraluminal spermatozoa. The degenerated cells increased rapidly and reached their greatest number 24h, and then decreased to smaller numbers until 48h. These degenerative changes were followed by the appearance of macrophages in the epithelium, which were first seen 12h after operation, becoming very frequent at one and 1.5 days, and then decreasing to become rare at 3 and 5 days. The macrophages phagocytosed the degenerated principal cells. The degenerated principal cells were also ingested and digested by undegenerated principal cells and basal cells. The intraepithelial mitotic figures had almost disappeared at 24h but were frequently observed at 2 days and more so at 3 days. They returned to normal numbers at 5 days. The volume of the initial segment was decreased to one third until the third day. The principal cells in this segment became similar in both light-microscopic appearance and ultrastructures to the cells in the next segment 5 days after efferent duct cutting. The changes were localized only in the initial segment.