Distribution of myelin basic protein (MBP) in the central nervous system (CNS) following injection into the cerebrospinal fluid (CSF) was studied by different qualitative and quantitative immunelectron -microscopic techniques. Endogenous MBP was present in myelin sheaths in injected as well as in control animals. After injection of exogenous MBP into CSF this protein was present in the subarachnoid space, on the surface of meningeal cells, on the surface of collagen fibers, in the basement membrane of the glia limitans, in vessel walls, and in the extracellular space of spinal roots. In meningeal veins, endothelial vesicles filled with peroxidase reaction product were found on the abluminal side of endothelial cells, in the endothelial cytoplasm and sometimes opening into the vascularllumen . In addition patchy staining of the luminal surface of endothelial cells was noted, indicating binding of antigen at this location. Quantitative immunelectron microscopy (an indirect technique with rabbit anti-MBP serum as primary layer and gold-labeled anti-rabbit IgG as secondary layer) revealed highly significant MBP binding on the luminal surface of endothelial cells after injection of this antigen into the CSF. The present results indicate that MBP, when liberated in CNS is transported through the blood-brain barrier and presented on the luminal surface of endothelial cells of the cerebral and meningeal veins. This observation may be important in interpretation of pathogenesis of initial inflammatory infiltrates in experimental allergic encephalitis (EAE).