Plasma insulin and blood glucose levels were measured in umbilical venous samples obtained by cordocentesis from 42 small-for-gestational-age and 68 appropriate-for-gestational-age fetuses at 17 to 38 weeks' gestation. In the appropriate-for-gestational-age fetuses plasma insulin and the insulin/glucose ratio increased exponentially with gestation, reflecting the progressive maturation of the pancreas. Maternal blood glucose concentration, not fetal plasma insulin, was the major determinant of fetal blood glucose concentration. In some small-for-gestational-age fetuses hypoinsulinemia and hypoglycemia were found; the two were significantly correlated. However, the fetal insulin/glucose ratio was lower in small-for-gestational-age fetuses than in appropriate-for-gestational-age ones, suggesting that hypoinsulinemia in a small-for-gestational-age fetuses is not only the result of hypoglycemia but also a consequence of pancreatic dysfunction. The degree of fetal smallness did not correlate with plasma insulin or with the insulin/glucose ratio. These findings suggest that insulin may influence fetal growth through its action on nutrient uptake and utilization but it is not the primary determinant of fetal size.