The concentrations of plasma glucose, free fatty acids, insulin, growth hormone, and placental prolactin in subhuman primate fetal and maternal plasma were examined following intravascular administration of insulin and glucagon to the fetus and mother. The neonatal plasma responses to these same stimuli were also examined. Fetal plasma glucose concentrations were minimally altered by direct fetal insulin injections, whereas neonatal glucose levels declined with similar injections. In both instances, however, plasma free fatty acid levels declined following insulin. When the amount of insulin given the fetus was increased, fetal plasma glucose concentrations did decline. Combined intravascular insulin injections and infusions in the mother were associated with a disappearance of the initial maternal to fetal plasma glucose concentration gradient and a nearly parallel fall in both maternal and fetal plasma glucose levels. It was concluded that insulin was biologically active in the fetus. Obtunded fetal plasma glucose responses to direct fetal insulin administration may be a function of placental transfer of glucose from the maternal pool. Maternal plasma placental prolactin and fetal plasma growth hormone levels were unchanged in the presence of sustained maternal and fetal hypoglycemia. However, neonatal plasma growht hormone levels did increase in response to hypoglycemia. The observed bidirectional placental barrier to transfer of radioisotopically labeled growth hormone indicated that fetal plasma growth hormone was solely of fetal origin. These data suggested further that a change in the growth hormone-releasing mechanism may occur from fetal to neonatal life. Direct maternal intravascular glucagon administration led to augmentation in both maternal and fetal plasma insulin and glucose levels. Direct fetal glucagon injections enhanced both maternal and fetal plasma insulin levels. These simultaneous changes in both plasma pools were consistent with the demonstration of a bidirectional placental transfer of radioisotopically labeled glucagon. The role of endogenously produced glucagon in these studies remains to be clarified.