The role of endothelial cell anionic surface charge in the function of the blood-nerve barrier (BNB) was investigated in 16 Sprague-Dawley male rats. The polycation protamine sulphate (PS) was administered parenterally in 8 animals to neutralize the endothelial anionic charge. The effect of such neutralization on the permeability of the BNB to intravenously injected horseradish peroxidase (HRP), and on anionic sites labelling with cationic colloidal gold (CCG) and cationic ferritin (CF) were studied qualitatively and quantitatively. Eight control animals were similarly studied after parenteral administration of saline or buffer instead of PS. PS-treated animals showed a pronounced opening of the BNB to HRP, and a significant reduction of endothelial cell surface labelling with cationic probes. The association of a reduction in anionic sites with the breakdown of the BNB supports the view of an endothelial surface charge filter acting as a contributing factor to the normal function of the BNB. The possible role of charge in transcellular and paracellular transport routes is discussed.