In the streptozocin-induced diabetic rat, the placenta is larger and the fetus is smaller than normal. To study cellular differences that might contribute to the size and functional disparity between diabetic and control placentas, a light- and electron-microscopic analysis was performed on 14-, 18-, and 22-day (term) control and diabetic placentas. Diabetic placentas, especially later in gestation, were marked by the presence of large numbers of glycogen-distended cells in the basal zone. Within the placental labyrinth, the trophoblastic layers of the interhemal membrane were significantly thicker in the diabetic placentas on days 18 and 22, and large accumulations of liid droplets were present, especially in the inner two trophoblastic layers. In normal placentas there is a marked thinning of the placental barrier in the labyrinth by day 22 of gestation, making the thickness of the labyrinthine layers in age-matched diabetic placentas even more impressive. Finally, the labyrinth of 22-day diabetic placentas contained more glycogen and rough endoplasmic reticulum in the inner trophoblastic layer, a feature that is found in less-mature (18-day) control placentas. Thus, the diabetic placentas have a number of features that are consistent with functional immaturity/dysmaturity. Cytologic evidence confirms the presence of increased glycogen and lipid reserves in both the junctional zone and the cellular area between maternal and fetal blood.(ABSTRACT TRUNCATED AT 250 WORDS)