Epithelial and myoepithelial cells of rat mammary glands following administration of various hormones and surgeries were examined under an electron microscopy. 1) Following administration of estradiol or progesterone, the epithelial cells did not show significant changes, however the myoepithelial cells showed hypertrophic changes associated with finger-like radial processes toward the stroma. 2) Following administration of estradiol and progesterone, the epithelial cells showed accumulation of many lipid droplets throughout the cytoplasm and several protein granules in the apical protion of the cells. 3) Following administration of prolactin 25 IU/day, the epithelial cells contained many lipid droplets but not protein granules. 4) Following administration of estradiol and prolactin, the epithelial cells demonstrated many swollen rough endoplasmic reticulum and hypertrophy of the Golgi apparatus. The cells contained many lipid droplets throughout the cytoplasm and many protein granules particularly abundant in the cytoplasm near the duct lumen. 5) Following tripple operations (hypophysectomy, ovariectomy and adrenalectomy), the mammary glands showed marked atrophy. The epithelial cells showed degranulated swollen rough endoplasmic reticulum, several types of vacuoles and swollen mitochondrias with destruction of cristae.