Carbohydrate metabolism may be markedly altered in the pregnant patient with diabetes mellitus. How such changes in maternal physiology affect the placenta, a tissue of fetal origin, has been studied by measuring glycogen synthetase, glycogen phosphorylase, and glycogen concentrations in fresh term placental tissue from eight normal women delivered vaginally, eight normal patients delivered by repeat cesarean section, and fifteen patients with diabetes mellitus. Placental weights were also determined. Placentas of normal patients, whether delivered vaginally or by cesarean section, were extremely similar in all parameters studied. In contrast, the glycogen concentration in placentas from diabetic patients was significantly elevated (p < .001) although glycogenesis appeared limited by decreased levels (p < .01) of synthetase I. Phosphorylase a activity was increased significantly (p < .025) in placental tissue of diabetic patients as was total phosphorylase (p < .05). Placental weight was decreased in patients with retini-tis proliferans. These placental enzyme differences noted between the normal and diabetic patient suggest that insulin may play an important role in controlling glycogen metabolism in the human placenta. In diabetes mellitus, increased glycogen concentrations of the placenta may be due to prolonged hyperglycemia throughout gestation and that the elevated term placental glycogen may be responsible for the observed suppressed synthetase I formation and the stimulated phosphorylase a activity found in the diabetic term placenta.