In the late pregnant rat, blood glucose levels were lower and plasma RIA-insulin levels were slightly higher than in virgin animals. Oral and intravenous glucose tolerance tests produced parallel changes in blood glucose in both groups whereas plasma RIA-insulin increased more in the pregnant animals. Blood glucose levels after either low (0.1-1 IU/Kg) or high (10 IU/Kg) doses of intravenous insulin decreased more slowly and less in pregnant than in virgin rats. Feta! blood glucose levels were not affected by maternal insulin treatment. Results show that in the unanaesthesized late pregnant rat both insulin sensitivity and responsiveness decreased and it is proposed that this insulin resistance may represent a mechanism to delay disposal of ingested nutrients by maternal tissues, ensuring their availability to the fetus.