During the perinatal development of rats, pancreatic endocrine cells (B, A and D cells) were quantitated morphometrically and plasma insulin, glucagon and somatostatin were measured. Moreover, intrauterine growth retardation (IUGR) rat fetuses were induced by uterine artery ligation and at the 21st day of gestation their volume density of pancreatic endocrine cells and plasma hormone levels were compared with that of normal fetuses. At the 16th day A cells were more numerous than B cells. But after then, the volume density of B cells increased rapidly. Plasma insulin also increased in the fetal period and was very high at the late fetal day. Just after birth plasma insulin decreased immediately and plasma glucagon increased and reached a very high peak. The volume density of D cells was much lower than that of the other cell types, and plasma somatostatin did not change remarkably throughout the perinatal period. In the IUGR rat fetuses the volume density of B cells was significantly lower than that of controls. In addition, plasma insulin was lower in the IUGR group, whereas plasma glucagon was higher. These results suggest that the pancreatic endocrine system, especially insulin and glucagon play some important roles in fetal development and postnatal metabolic changes.