Three types of prolactin (PRL) cells of the rat anterior pituitary were recognized by immunogold electron microscopy: Type I cells are characterized by irregularly shaped large secretory granules (500 nm in maximum diameter); Type II cells contain spherical granules of varying size (150-250 nm); and Type III cells are immature forms with a narrow cytoplasm and small round granules (100 nm). In the male adult pituitary, Type II cells occur most frequently (48%), Type I cells slightly less frequently (46%), while Type III cells are very rare (6%). In the female gland, Type I cells exceed 90% of the total PRL cells, Type II cells make up 7% and Type III cells comprise 3% through the 4 day estrous cycle. Type I cells undergo marked changes in ultrastructure during the estrous cycle. This cell type shows ultrastructural signs of elevated secretory activity during the proestrus and estrus. The postnatal development of PRL cells was also studied. At 8 days after birth, immunoreactive PRL cells are mostly Type III cells (more than 80%), Type II cells are much fewer (17%), and no Type I cells can be observed. At 21 days the female pituitary fully matures with regard to PRL cells. From the 33rd day, the male PRL cells show characteristics of the male pituitary. Transitional features changing from Type III into Type II or from Type II into Type I were observed. It is thus suggested that Type III cells are primitive immature cells which may give rise to mature, functionally active Type II or Type I cells. The Type III cells may frequently undergo mitosis.