Both androgen and estrogen receptors (AR and ER) are present in the rabbit epididymis. We have used the sucrose gradient method to examine receptor sedimentation properties, receptor concentration, and distribution of receptors among the caput, corpus, and cauda of the epididymis to determine changes that occur in these parameters as the animals age. The 9S form of the ER is present in all three epididymal segments of the immature rabbit, with the highest concentration occurring in the cauda. The 8.2S form of the AR is also present in all three segments of the immature epididymis, with the highest concentration occurring in the caput. Short-term castration (3 days) leads to an increase in the amount of both AR and ER detected. ER are present in all segments of the immature epididymis at higher concentrations than AR. The functional 9S form of the ER disappears as the animals mature, the result of a tissue-specific protease that our laboratory previously has shown proteolyzes ER to a non-DNA-binding 3.8S form. Long-term castration (3 mo) of adult rabbits results in the reappearance of the 9S form of the ER in all segments of the epididymis. The reappearance of the 9S form of the ER is also seen in animals castrated for 1 mo, but not in those castrated for 2 wk. Administration of testosterone once daily for 2 wk to adult animals castrated for 6 wk results in the disappearance of the 9S form of the ER and the reappearance of the 3.8S form, suggesting that the tissue-specific protease is androgen-dependent. In this way, circulating androgens may play a role in regulating the concentration and form of the ER in the rabbit epididymis. There is little change in the concentration or distribution of AR in the epididymis of adult rabbits castrated for 3 mo as compared to those castrated for 3 days. This implies that circulating androgens are not required for maintenance of AR in the epididymis. Our data demonstrate that there are temporal differences in the presence and concentration of ER and AR in the epididymis and suggest that there is a differential, age-dependent regulation of the development and function of the epididymis by androgens and estrogens.