To determine the temporal sequence of pregnancy-induced changes in insulin action and secretion, awake midpregnant (11-12 days) and late pregnant (19-20 days) rats underwent a two-step euglycemic hyperinsulinemic or a hyperglycemic clamp study after a 24-h fast. During euglycemia, insulin-stimulated increments in glucose uptake and clearance in midpregnant rats were reduced by 60-70% at the lower dose (insulin approximately 360 pM) and by 20-30% at the higher dose (insulin approximately 1,750 pM; P < 0.01 vs. virgin controls). Insulin action was also diminished in late pregnant rats. However, the magnitude of resistance did not increase. Insulin-mediated suppression of glucose production was only minimally impaired in midpregnancy. In contrast, glucose production was virtually unchanged in late pregnancy, even at the highest insulin dose. During hyperglycemia, insulin responses in late pregnancy were markedly increased 5-fold above controls and 2.5-fold above midpregnant rats (P < 0.05). We conclude that rat pregnancy is characterized by the early appearance of peripheral insulin resistance. As pregnancy progresses toward term, marked hepatic insulin resistance and insulin hypersecretion develop, whereas peripheral insulin resistance demonstrates negligible changes. These data imply that insulin hypersecretion during late pregnancy is most closely linked to hepatic insulin resistance, at least in 24-h-fasted animals.