The effects of different androgen and testicular fluid levels on the protein synthesis and secretion of the epididymis of mice and rats were examined. In mice, the in vitro protein synthesis and secretion of five epididymal segments were measured from 3 days up to 8 weeks after efferent duct ligation. During the entire period, alterations in the rate of protein secretion per milligram tissue were small. At 8 weeks, the mean rate of protein synthesis per milligram ligated epididymal tissue was 80% of that of the control side. As a consequence of the weight loss of the ligated epididymis, however, the protein secretion per organ can be estimated to be reduced by 50%. Changes in the protein profile were only found in the proximal segment, where a 40-kd protein appeared and a 29-kd protein disappeared. In rats, the effects of efferent duct ligation were studied in vivo for up to 8 months. Structural changes were present both in the proximal and in the distal epididymis. The most conspicuous change in the protein profile of secretory proteins was the disappearance of a 27-kd protein from the proximal segment. In the distal epididymis, a 32-kd protein was no longer secreted. In mice, the effects of castration on the profile of secreted proteins demonstrated that, without androgen stimulation, some proteins are still secreted 6 weeks after castration. Administering low or high doses of testosterone propionate to castrated mice resulted in almost similar profiles of secretory proteins. However one protein secreted in the proximal epididymis was preferentially stimulated by the high dose of testosterone propionate.