Previous studies of the estrogen receptor-alpha knockout (αERKO) in the male mouse demonstrate that the rete testis and efferent ductules are targets of estrogen. Because the αERKO mouse lacks a functional estrogen receptor alpha (ERα) throughout development, it was not known whether the morphological and physiological abnormalities observed in the αERKO male were due to developmental defects or to dysfunctions concurrent with the lack of ERα in the tissue. This study was designed to determine if treatment of normal wild-type (WT) mice with the pure antiestrogen, ICI 182,780, (ICI) could reproduce the morphological characteristics seen in αERKO mice. Thirty-day-old male mice were treated for 35 days with either castor oil or ICI. Age-equivalent αERKO mice were used for comparison. Light microscopic examinations of the reproductive tracts revealed dramatic changes in the efferent ductules of treated mice: a 1.7-fold increase in luminal diameter, a 56% reduction in epithelial cell height, a 60% reduction in brush boarder height of nonciliated cells, and an apparent reduction of the number of observable lysosomes and endocytotic vesicles. Testes of ICI-treated mice showed swollen rete testes area (6.5 times larger than control) and a 65% reduction in rete testis epithelium height. However, there were no significant changes in body and testis weights. These results indicate that ER blockage with ICI in WT mice results in morphological changes of the efferent ductules resembling those seen in αERKO siblings of the same age. Based on this study, we conclude that ERα has a functional role in the mouse reproductive tract and the aberrant morphology observed in the efferent ductules of the αERKO mouse is likely the result of a concurrent response to the lack of functional ERα, and not solely due to the lack of ERα during early developmental times.