SummaryFetal brain development was investigated near term in guinea pigs rendered diabetic with streptozotocin. The liver and the placenta were used as reference organs. Compared to controls, those fetuses from diabetic animals had normal cerebrum and cerebellum weights, but higher liver and placenta weights in relation to fetal weights. Although liver and placenta cell number (DNA content) was unchanged, it was significantly increased in the fetal cerebrum and cerebellum of diabetics. Although the tissue protein concentration was decreased in the liver and the placenta, it was unchanged and even increased in the cerebrum and cerebellum, respectively. The concentration of myelin (cerebroside-sulfatide) was unchanged in the cerebrum, but it was increased in the cerebellum of diabetic animals.These data suggest that diabetes has a growth-promoting effect on the fetal brain cell number. Furthermore, differences in the protein content between fetal organs may reflect abnormalities in protein metabolism which do not affect the brain during diabetic pregnancies.