The concentrations of plasma free fatty acids (FFA) and glucose were measured in fetuses, and newborns whose mothers were exposed to various procedures (i.e., intravenous glucose administration, cesarean section, abnormal gestation) and stresses (i.e., labor and hypoxia). The results demonstrate that the regulation of fetal plasma FFA is controlled by the same mechanisms as those of the pregnant adult. The fetus is however, unable to respond to a high plasma glucose concentration as the pregnant woman does (i.e., by a decrease in plasma FFA concentration). This difference is probably due to a limited supply of fetal insulin and/or to the saturation of the enzyme systems synthesizing lipids in fetal tissues. Pathologic conditions of gestation and stresses in utero have no effect on the fetal plasma FFA concentrations. This is considered as an expression of fetomaternal homeostatic ability. Increase in fetal plasma FFA concentrations takes place after birth. It is triggered by the transition from the intra- to the extrauterine environment. The activity of the newborn sympathetic system coupled to a change in glucose availability postnatally is the basic mechanism which stimulates the increase in plasma FFA concentrations.