Placentas of rats with diabetes mellitus induced by streptozotocin are investigated. Histologically, in the spongiosa zone we find dilated and congested maternal sinus as well as cysts of different size and number. These cysts contain granular eosinophilic material and cytotrophoblastic cells with large amount of glycogen. In our opinion, these cysts are large glycogen islets of the spongiosa zone respectively their remnants. However, similar findings we see in smaller extension in normal rat placentas too. The glycogen content in placentas of diabetic rats is in all localisations higher than in control cases. Comparable histological changes like in diabetic human placenta such as placental disturbances of maturation we don't find in the placental labyrinth of diabetic rats.