The proliferative activity of the rat corpus luteum was studied on days 2, 3, 6, 9, 12, 15, 17, 19 and 21 of pregnancy. Proliferating cells were detected by the immunohistochemical demonstration of DNA-incorporated 5-bromodeoxyuridine (BrdU) and by the presence of mitoses. Steroidogenic luteal cells showed two proliferative waves on days 12–15 and on day 21, when relatively abundant BrdU-labeled and mitotic cells were observed. These cells were clearly distinguishable from non-steroidogenic cells by their round nuclei and large polygonal cytoplasm. The proliferative activity on days 12–15 was coincident with an increase in the size of the cells and in progesterone concentrations. On the other hand, the proliferative activity of non-steroidogenic luteal cells (especially endothelial cells of the blood and lymphatic vessels) followed a different pattern. These cells intensely proliferated on days 2–3 of pregnancy and this proliferative activity was significantly higher than that observed in non-pregnant rats on metestrus and diestrus. A new proliferative wave was observed on days 12–15, in association with the increase in the proliferative activity of steroidogenic cells. The presence of both BrdU-labeled and mitotic steroidogenic luteal cells provides evidence that these cells do proliferate and that both hypertrophy and hyperplasia are involved in the increase in the parenchyma of the corpus luteum during pregnancy. Also, the results suggest that different mechanisms are involved in the regulation of the proliferative activity in the corpus luteum at different times during pregnancy.