We hypothesized that the concentrated urea and NaCl solutions may opened the blood-brain barrier to the horseradish peroxidase and trypan blue-albumin complex by shrinking barrier cells and opening up spaces between them. The experiments were carried out on 30 healthy, adult rats. Two experimental groups were used. First, intracarotid perfusion of anesthetized rats were prepared by exposing and catheterizing the left common carotid artery. A test solution of 3.4 M, 3.0 Osm urea and 0.87 M, 1.6 Osm NaCl, was perfused manually for 30 sec. in a cranial direction so as to expel the blood from the pial arterioles of the exposed brain. The pressure, which was not measured, varied between what was required which to expel blood from both arterioles and venules. Five milliliters of the test solution usually were used. Horseradish peroxidase and trypan blue was injected intravenously or through the carotid artery after perfusion. Threshold of barrier damage due to the intracarotid substance was defined as the lowest osmotality which produced obvious blue staining of the brain both on surface observation and coronal section. The effect of a substance was defined as reversible if a threshold concentration did not produce blue staining when the dye was injected 30 min following perfusion. Second, we applied a concentrated solution of 3.0 Osm urea and 1.6 Osm NaCl to the pia-arachnoid of the cerebral cortex, to study the barrier to the intravascular horseradish peroxidase and trypan blue-albumin complex. Hyperosmotic solution of 3.0 Osm urea and 1.6 Osm NaCl, either infused into one internal corotid artery or applied topically to the pia-arachnoid surface of the brain of rats, results in the opening of endo thelial tight junction through which horseradish peroxidase passes from blood to the basal menbrane and astrocytes and neurons. The evidence for this opening of the blood-brain barrier to protein is the entry of peroxidase into the neurons. It was postulated that sufficiently high concentrations of electrolytes or relatively lipid-insoluble non-electrolytes such as urea, osmotically pulled water from the cerebral endothelial cells resulting in their shrinkage. The shrinkage, in turn, was believed to open the tight junction between continuous endothelial cells so that the dyeprotein complex could pass through the junction from blood to neurons. The present study shows that these tight junctions, the sites of the barrier to neuron movement of protein, are indeed opened by the osmotic action of urea or NaCl.