Endothelium-dependent dilation, produced by applying acetylcholine (ACh) to pial arterioles, was unaffected after 6 mo of a diet with zero vitamin E or 8 mo of a vitamin E-enriched diet. The enriched diet did not affect constriction produced by topically applied NG-monomethyl-L-arginine, an inhibitor of the synthesis of endothelium-derived relaxing factor (EDRF). EDRF mediates the response to ACh and is a basally released dilator and antiplatelet paracrine substance. Endothelial injury produced by a helium-neon laser and Evans blue technique eliminates the response to ACh, but in vitamin E-enriched mice the response to ACh was unaffected by the injury. More prolonged exposure of the laser induces platelet adhesion/aggregation at the injured site. A significantly longer exposure to the laser was required to initiate adhesion/aggregation in vitamin E-enriched mice. Because effects of endothelial damage in this model are mediated at least in part by singlet oxygen produced by injured tissue (W.I. Rosenblum and G.H. Nelson, Am. J. Physiol. 270 (Heart Circ. Physiol. 39): H1258-H1263, 1996.), we conclude that the antioxidant, radical-scavenging actions of vitamin E explain the protective action of the vitamin E-enriched diet. However, raising vitamin E levels did not protect against putative adverse effects of normally occurring oxidants.