The effect of histamine administered via the common carotid artery on the transport processes of brain capillaries was investigated in rats. The fine structure of endothelial cells and the glial end-feet system was studied by electron microscopy and the serum albumin was visualized for light microscopy by the peroxidase-antiperoxidase (PAP) immunohisto-chemical reaction. Sixty microgram per milliliter histamine enhanced the penetration of serum albumin into the capillaries while the number of pinocytotic and coated vesicles significantly increased in the capillary endothelium. Oedematous swelling of the glial end-feet system was also observed. The stimulatory effect of histamine on the transcapillary transport could not be inhibited by a histamine H1-receptor antagonist, mepyramine. By contrast, metiamide, a histamine H2-receptor antagonist prevented both ultrastructural changes and albumin penetration in the brain capillaries to occur.