The ultrastructure of the placentae on day 20 of gestation was studied in rats made diabetic by streptozotocin injection on day 13 of gestation. In the placentae of control rats most of the glycogen was found in the glycogen cells, while some of it was localized to the labyrinth trophoblastic layers. In the diabetic rats a marked increase in glycogen content, together with higher numbers of glycogen cells in the junctional zone, was seen. Glycogen was also stored in other cell types of this zone, as well as in all cell types of the placental labyrinth of the diabetic animals.