The effects of pregnancy and dietary status upon the blood sugar response to exogenous insulin in the rat were evaluated. Unanaesthetized 19-day pregnant, postpartum, and age-matched virgin rats were challenged with intravenous insulin (10 U/kg) after unrestricted access to food or fasting for 48 hours. Appropriate control studies were instituted to correct for the effects of 'handling' upon blood sugar. Fed as well as fasted pregnant rats displayed diminished absolute hypoglycaemic responses to insulin and attenuated rates of blood sugar fall. The relative resistance of pregnant rats to the blood sugar lowering actions of insulin was documented during group comparisons with virgin rats as well as upon reexamination of the same animals postpartum.