Evidence is accumulating that suggests that increased permeability of the BBB to blood-borne proteins is favorable for the development of neuropathologic changes such as amyloid angiopathy and formation of amyloid plaques in the AD brain. To study this problem, we applied a quantitative immunocytochemical procedure that enables evaluation of the barrier function of brain microvasculature to endogenous albumin. This procedure was successfully used on scrapie-infected mice, which represent a unique animal model enabling study of an interrelation between BBB function and deposition of amyloid within vascular wall and neuritic plaques. Biopsy specimens obtained during neurosurgical procedures (tumors and dementia) were also examined. Our observations indicate that (1) the vast majority of brain microvessels in scrapie-infected mice and in demented individuals show normal features of the BBB; (2) only those microvascular segments directly surrounded by amyloid plaques or representing amyloid angiopathy show increased permeability to endogenous albumin; (3) numerous immunosignals over the amyloid deposits in plaques and in the wall of angiopathic vessels suggest the affinity of extravasated albumin to the amyloid material.