The influence of pregnancy on serum glucose, serum insulin, and plasma glucagon concentrations was studied in eight normal women and 14 nonobese gestational diabetics. Each normal individual was subjected to an oral glucose tolerance test in midpregnancy, and all subjects were investigated late in pregnancy and again four to six weeks postpartum. As compared with the postpartum values, fasting glucose concentration decreased during gestation in the normals and increased in the gestational diabetics, but the changes were small and insignificant. In contrast, fasting insulin concentration increased equally and significantly in pregnancy in both groups. Likewise, the glucagon concentration was enhanced in the fasting state in both groups in pregnancy. However, the molar insulin-to-glucagon ratio was significantly elevated in both normals and gestational diabetics. In late pregnancy the magnitude of the insulin response to oral glucose (i.e., the incremental insulin area above fasting baseline) was equally and significantly enhanced in the normals and the gestational diabetics. However, when the insulin response during the first 60 minutes of the OGTT was expressed per unit of glucose stimulus (i.e., the delta insulin/delta glucose ratio) a significantly higher mean response was found in the normal pregnants than in the gestational diabetics. In pregnancy and postpartum, plasma glucagon always decreased to levels significantly below fasting levels after glucose ingestion. In normal midpregnancy the degree of suppression of glucagon was close to that of postpartum, whereas an exaggerated and prolonged suppression was found in late pregnancy in the normals as well as the gestational diabetics. These findings indicate that plasma glucagon and serum insulin concentrations are profoundly influenced by pregnancy. As the changes, however, lead to an increased insulin-to-glucagon ratio, the diabetogenicity of pregnancy is not explained by this relationship.