The ultrastructural characteristics of blood vessels within 18 metastatic neoplasms of the brain have been studied. The abnormal permeability to protein-bound tracers was found to be related to defective capillary endothelium. Abnormalities include gap functions, fenestrations of membranes, and open normal junctions. The abnormal capillaries had the character of capillaries from current tissue. Endothelium cell abnormalities were not found in surrounding edematous brain.