To study the proliferation and differentiation of pituitary corticotrophs, we administered bromodeoxyuridine (BrdU) to pregnant rats at 15.5–21.5 days of gestation and to rat pups at 3, 7, and 28 days after birth. The pituitary sections of fetuses and pups were consecutively immunostained with anti-BrdU and anti-adrenocorticotropic hormone (ACTH) to detect proliferating cells and corticotrophs, respectively. The number of cells labeled with BrdU, ACTH, or both were counted. The diameters of their nuclei and the volume of the pituitary were measured. The BrdU-positive cells were around 76,000–96,000/mm³ during the period studied. The corticotrophs were first detected in the fetus at 15.5 days and they increased during the fetal and postnatal periods. The double-labeled cells were first detected in the 17.5-day fetus. They increased markedly at 19.5 days and comprised about one-quarter of the corticotrophs that increased in 24 h at this stage. These results indicate that: (1) at 15.5–18.5 days the corticotrophs were derived almost exclusively from undifferentiated cells; (2) during the later fetal and early postnatal periods, the proliferation of existing corticotrophs contributed, at least in part, to their increase; (3) About 1/20 of proliferating cells differentiated to corticotrophs when their increase was required.