Pial arterioles of mice are studied by in vivo TV microscopy. Focal endothelial injury is produced by a laser/Evans blue technique. Moderate damage results in local platelet aggregation. Very slight damage, without electron microscopic evidence of injury, results in loss of many endothelium derived vasoactive factors. These include "EDRFs" for acetylcholine, bradykinin and calcium ionophore, and "EDCFs" for histamine and serotonin. In the cases of acetylcholine, histamine and serotonin, each agonist possesses an additional opposing action which is independent of endothelium. The latter action is unmasked by the endothelial injury. The balance between simultaneously acting endothelium dependent and endothelium independent actions is a determinant of the response to an agonist with two opposing actions. This balance is partly dependent upon initial tone. Thus the effect of the agonist depends on initial tone. One of the determinants of initial tone may be basal release of one or more EDRFs or EDCFs. Evidence in pial arterioles for the basal release of EDRF for acetylcholine, comes from our data showing that L-NMMA constricts these arterioles. L-NMMA is a known inhibitor of synthesis of "classical" EDRF from L-arginine. The response to L-ARG is relaxation. Both the response to L-NMMA and the response to L-ARG are abolished by laser/dye injury of the endothelium. Thus these agents are really acting via an endothelial mechanism in brain arterioles, just as has been reported for their actions in conductance vessels. Finally mild injury associated with loss of "EDRFs" is also accompanied by a reduced ability of pial arteriolar endothelium to repell activated platelets.