Estradiol benzoate was injected into virgin rats, at a concentration of 50 microgram/day, for 7 days. The animals were fixed by perfusion and the mammary glands prepared for electron microscopy. Many myoepithelial cells in the mammary glands were altered as a result of the estradiol treatment. The affected cells had a markedly reduced number of myofilaments that were restricted to a specific region of the cytoplasm. Large foot-like projections with many vesicles extended from the myoepithelial cells into the surrounding connective tissue. The altered cells also had more organelles, suggesting that their metabolic activity was increased. Nuclei in the altered cells had deep infoldings that may reflect an increase in nuclear/cytoplasmic exchanges. The results of this study indicate that estradiol causes specific morphological alterations in myoepithelial cells of the mammary gland.