We hypothesized that BBB is impaired in rat model of streptozotocin-induced diabetes and can be sealed by poly(amido)amine dendrimers G4.0 (PAMAM G4), which reveal anti-glycation activity. The BBB permeabilization was monitored in rats with the 60-day streptozotocin-diabetes and non-diabetic animals, using three fluorescent dyes (given intraperitoneally) differing in molecular weight: fluorescein, fluorescein isothiocyanate (FITC)-dextran and Evans blue. All animals were administered for 2 months with either PAMAM G4 dendrimer or placebo. The fluorescence intensities of the injected fluorescent markers were recorded in the homogenates of selected brain regions. The highest accumulations of the used fluorescent dyes were observed for fluorescein, predominantly in thalamus, hippocampus, frontal cortex, striatum and cerebellum. FITC-dextran leaked to much smaller extent, however, higher permeabilization for FITC-dextran was revealed in pons-medulla oblongata, frontal and parietal cortex of diabetic compared to control animals. Evans blue leaked very slowly into striatum and pons-medulla oblongata in diabetic rats. The treatment of diabetic animals with PAMAM G4 significantly reduced blood glucose concentration and hallmarks of late diabetic complications, compared to non-treated diabetic animals. PAMAM G4 significantly reduced diabetes-induced permeabilization of BBB, which remained in line with the reduced blood glucose and the amelioration of the biochemical hallmarks of severe hyperglycaemia.