We can produce platelet aggregation in the pial or mesentric microcirculation by illuminating a microscopic field containing such vessels with an appropriately filtered mercury lamp in the presence of intravascular sodium fluorescein (4, 5). This phenomenon appears to be a result of endothelial damage produced by the light in the prescence of the dye (5). Pretreatment of the animal with either aspirin or indomethacin inhibits the aggregation in the pial arterioles (5), but enhances aggregation in the mesenteric arterioles (6). During the course of our published studies concerning aggregation we observed arteriolar constriction in the mesentery and dilation in the pia, the latter being augmented and the former being reduced by aspirin and indomethacin. These observations are the subject of this brief report.