Cerebral blood flow during and after spreading depression (SD) was studied in rat brain by quantitative autoradiography. The rise of cortical blood flow during SD was followed by 20 to 25% reduction belwo normal after SD, lasting at least an hour. Blood flow in the putamen, thalamus, and hippocampus did not change at any time during or after SD. Previous measurements of cerebral blood flow in humans showed that migraine attacks may be accompanied by wavelike spreading oligemia (Olesen et al, 1981). We speculate that the spreading oligemia of migraine may be a phenomenon physiologically related to the present finding of an oligemia after SD.