The effects on the proximal caput epididymidis of efferent ductule ligation were studied. After 6 h there was a slight increase in autophagocytosis in the initial segment proper (1A). After 12 h this segment showed cell death and cytoplasmic regression and after 24 h large amounts of necrotic cells and focal degeneration in the epithelium, and desquamated cells and débris in the lumen. After 48 h the epithelium was lower, but degeneration less obvious. Segments 1B, 1C and 2 showed marked cell death and cytoplasmic degeneration. This was more evident after 3 days, especially in 1B. After 5 and 7 days some degeneration was still seen, but essentially regional differences had disappeared, leaving a lower, inactive epithelium throughout. Except for an increase in glycogen granules in segment 1C after 48 h, marked focal degeneration always appeared before general cytoplasmic alterations. Sham operation produced no changes. The regression might be caused by the cut supply of androgen binding protein or a growth factor in rete testis fluid