To assess whether placental DNA, glycogen, and fat are affected by diabetes mellitus and whether the changes correlate with the disturbance in maternal fuels, placentas were examined at delivery in women with normal carbohydrate metabolism, gestational diabetes mellitus, and Classes B to F diabetes and in rats rendered diabetic by the administration of streptozotocin 2 weeks before mating. Plasma glucose values during late pregnancy were higher in patients with Classes B to F diabetes than in the patients with gestational diabetes mellitus so that they were judged to have more severe metabolic disturbances; rats were also divided into groups with "mild" or "severe" diabetes on the basis of blood sugar. In rats, as in humans, diabetes tended to increase placental mass, DNA, glycogen, and lipids. However, the relative changes in glycogen and fat exceeded the alterations in mass and DNA, especially in those with more severe diabetes, so that a true increase in glycogen and fat per placental cell may have occurred. Thus, placental glycogen/DNA and placental triglycerides/DNA significantly exceeded control values in patients with Classes B to F diabetes and in rats with severe diabetes but not in women with gestational diabetes mellitus or rats with mild diabetes. Total placental triglycerides and total placental glycogen were significantly correlated (r = 0.952; p less than 0.001) in rats with experimental diabetes, which suggests that these alterations in placental composition during late pregnancy may share communal dependencies.