The effect of a dietary supplement of an aldose reductase inhibitor (ponalrestat) or of myo-inositol on sodium transport into the rat brain and on concentrations of saccharide and polyols in cortical brain tissue and sciatic nerve was investigated in control rats and in streptozotocin-diabetic rats after a diabetes duration of 2 weeks. In untreated diabetes, the neocortical blood-brain barrier permeability for sodium decreased by 28% (3.4 +/- 0.4 vs 4.7 +/- 1.6 x 10(-5) ml/s g, mean +/- SD) as compared to controls. Levels of glucose, sorbitol and fructose increased in brain as well as in nerve tissues, whereas myo-inositol depletion was not demonstrable. Ponalrestat treatment of diabetic animals had no effect upon the decreased neocortical blood-brain barrier permeability to sodium (3.5 +/- 0.9 vs 4.7 +/- 1.1 x 10(-5) ml/s g) despite normalization of brain and nerve content of sorbitol and fructose. Myo-inositol supplementation of diabetic rats normalized sodium passage into the brain (4.2 +/- 1.1 vs 4.4 +/- 0.5 x 10(-5) ml/s g). Brain concentrations of monosaccharides and polyols were normalized as compared to the myo-inositol treated control group and nerve concentrations of glucose, sorbitol, and fructose were significantly increased. Myo-inositol treatment leads to a normalization of blood-brain barrier permeability; it is suggested that myo-inositol exerts a restituting effect upon Na+/K+-ATPase activity of the cerebral endothelial cells.