Insulin receptor binding to monocytes and erythrocytes was measured at the time of diagnosis and again postpartum in 23 normal weight gestational diabetics. In 16 women, insulin receptor binding was studied after a period of dietary treatment. In patients diagnosed early in pregnancy, an increase in insulin binding to both monocytes and erythrocytes was observed during dietary treatment (p less than 0.001), while no similar changes were observed in patients diagnosed later in pregnancy. Postpartum, insulin receptor binding to erythrocytes decreased in all women compared to the time of diagnosis (p less than 0.01) while insulin binding to monocytes either decreased (p less than 0.001) or remained unchanged depending on the time of diagnosis. Insulin binding was similar in those women who remained glucose intolerant and those who returned to normal glucose tolerance postpartum. The findings indicate that factors other than insulin receptor binding are involved in the pathogenesis of gestational diabetes.