The existence of three morphologically different types of GH cells in the rat anterior Pituitary glands has been reported previously. Among these three types, Type III which contains the smallest granules has been considered to be immature, because this type occurs more frequently in neonatal rats than in adults. In this study, the prenatal development of GH cells in the rat fetus was observed by immunoelectron microscopy. In the rat fetus at 18.5 days of gestation, Type III cells were most numerous (48.5%), followed by Type II cells (45.5%). Type I cells were almost absent from these fetal pituitaries. At 20.5 days Type II cells exceeded Type III in frequency, and Type I cells also increased to about 35%. These results indicate that Type III cells are the most immature type of GH cells, and might transform into Type II and, in turn, to Type I, as the rats mature. Images indicating active secretory functions such as granule formation in the Golgi apparatus and/or GERL, exocytosis and crinophagy were observed in the GH cells even in the fetal stage.