The specific activities of three acid phosphatases were followed in the epididymis of growing rats. All activities were constantly rising but revealed two steeper parts probably corresponding to the increase of androgen secretion and the arrival of spermatozoa in the epididymis. Simultaneously an increase in the histochemical staining was obtained in all epididymal segments. Both biochemical and histochemical studies showed that castration reduced the activity of all acid phosphatases and this could be restored with testosterone-estradiol treatment. Estradiol had no effect on the activities in the castrated animal, but in the normal animal it caused an elevation of Enzyme I activity. In the histochemical study estradiol seemed to restore the acid phosphatase activity. Enzyme I reacted slower than the others to both castration and hormone treatment. Increased elimination caused by ligation of the epididymis caused a reduction in the activity of all enzymes within four days. It was greatest in the area of the corpus. This study, however, rendered no further elucidation for the assumption that acid phosphatases participate in the elimination of excessive and defective spermatozoa.