An investigation was conducted to determine whether lymphocytes were able to infiltrate into the brain parenchyma of normal rats in which hyperosmotic opening of the blood-brain barrier (BBB) had been created by internal carotid infusion of filtered 20% mannitol. Various numbers (1 x 10(6) to 3 x 10(8] of autologous lymphocytes were injected into the internal carotid artery, or via a tail vein, after BBB opening. Lymphocytes were detectable in the brain of rats subjected to high-grade BBB opening and injected with a high number of lymphocytes. Subsets of lymphocytes were examined by immunohistochemical staining using OX19, W3/25 and OX8 monoclonal antibodies. There was no remarkable difference in number between W3/25- and OX8-positive cells. From these results it was suggested that the intensity of BBB opening and the number of lymphocytes flowing in brain vessels are related to the passage of lymphocytes through the BBB.