The mammary gland of lactating rats was observed with the electron microscope. The cytoplasm of glandular cells contains parallel-arranged cisternae of well-developed rough endoplasmic reticulum. The Golgi apparatus is also well developed and contains small dark secretion granules of a protein nature. In the basal cytoplasmic area small fat droplets occur in the vicinity of an accumulation of tubular or vesicular elements of smooth endoplasmic reticulum, which is thought to be correlated with the elaboration of fat droplets. Fat droplets push up the luminal surface membrane, and protrude into the lumen. The plasma membrane constricts the cytoplasm slightly below the fat droplet. Some proteinaceous secretion granules encased in vacuoles are contained in the cytoplasmic husk around the fat droplet. Therefore, the protein granules as well as fat droplets are released by the mechanism of apocrine secretion, in which a small amount of the cytoplasm and plasma membrane may be also extruded.