The carbohydrate metabolism of maternal liver, placenta, and fetal liver obtained from normal, pregnant, diabetic, and diabetic pregnant animals was measured in vitro. All of these alterations in the physiologic status of the animal produce complex alterations in the carbohydrate metabolism of the tissues studied. The simplest hypothesis which adequately accounts for the observed results is that the effects of pregnancy and diabetes are approximately additive in the maternal organism, and that the metabolic defects of the infant are primarily the result of an oversupply of glucose to the fetus in utero.