estrogen receptors are present in the epididymis of the rooster our recent discovery that testicular germ cells and epididymal sperm contain active p450 aromatase suggests that the reproductive tract may be a target for estrogen therefore the objective of this study was to determine if estrogen receptors er are present in the avian epididymis using immunocytochemistry northern blot analysis and in situ hybridization immunoperoxidase staining for er was found principally in nuclei of nonciliated epithelial cells of proximal and distal efferent ductules and the epididymis duct the ciliated cells also appeared to be slightly positive in the efferent ductules week immunostaining was also observed in the connective tissue of the epididymis duct immunostaining was more intense in epithelial cells of the efferent ductules than in epithelial cells of the epididymal duct of connective tissue cells strong specific hybridization signals for er mrna corresponded to the same areas exhibiting immunocytochemical localization the presence of er mrna in the epididymis was confirmed by northern blot analysis which showed a single band corresponding to approximately 78 kb similar to that found in chicken oviduct based on these data we suggest that the efferent ducts of the rooster are a primary target for estrogen and that estrogen may have a role in the regulation of avian epididymal function