Efferent ductules of the male reproductive tract contain high concentrations of estrogen receptors (ER), which are essential for the regulation of fluid reabsorption and maintenance of normal epithelial morphology. Treatments with the antiestrogen ICI 182,780 and 17beta-estradiol cause a reduction in ERalpha expression; however, the mechanisms governing the down-regulation are undetermined. In other tissues, the ubiquitin-proteasome pathway appears to have a dominant role in regulating ERalpha turnover, although in the efferent ductules, an abundance of epithelial lysosomes could also participate in protein turnover. To study this activity, the expressions of proteasome, ubiquitin, and markers for the endocytotic apparatus (early endosome antigen-1 [EEA1], clusterin, and cathepsin D) were examined in rat efferent ductules and initial segment of epididymis. Distinct cellular, subcellular, and regional distributions of these proteins were observed in the epithelial cells. A gradient of proteasome, ubiquitin, EEA1, and clusterin staining was seen in the efferent ducts, which decreased 30%-41% from the proximal zone to the terminal common duct. Antiestrogen treatment resulted in significant decreases in proteasome, EEA1, and clusterin in the efferent ducts. Localization of ubiquitin-proteasome and endocytotic pathway components suggests that differential regulation is required for protein degradation and turnover in efferent ductules and head of the epididymis.