A dual illumination technique was used to examine the behavior of parenchymal arterioles of the cortex during cortical spreading depression in six cats. K+ produced a wave-ring spread of optical density changes concentrically from the injection site. At locations where the wave was passing, arterioles first constricted at one or two spots (55±43% of control), forming a spindle-shape in the early phase of cortical spreading depression, and then markedly dilated (155±57% of control) within 1 min. The dilation started at the constricted spots, propagated bidirectionally and finally resulted in full-length dilation of the arteriole. Although it varied in magnitude and time-course, this arteriolar behavior was observed in all six cats. Despite these changes, no associated downstream tissue microvascular flow changes were discernable.