The work was performed on female rats to which alloxan was administered at immature age. Placental permeability was studied in female rats, healthy ones (170 fetuses), with prediabetes (124 fetuses), with latent diabetes (102 fetuses), and with manifest diabetes (112 fetuses). Permeability of placental membrane was assessed by means of the fluorimetric method of determination in the fetal organism of uranin, a stain administered into the blood of a pregnant animal. The results of the investigations demonstrated that in rats with alloxan prediabetes, latent and manifest diabetes permeability of placental membrane showed a significant elevation (P less than 0.001). There was revealed a direct relationship between the extent of the insular insufficiency and permeability of the placental membrane.