Unilateral cerebrovascular occlusion was produced in heparinized rats within seconds after injection of microgram quantities of sodium arachidonate into the internal carotid artery. Electroencephalographic activity over the affected cerebral hemisphere became attenuated, and cerebral blood flow decreased by half. A neurologic syndrome, including ipsilateral blindness and contralateral sensorimotor deficits, resulted from occlusion of the microvasculature by platelet thrombi. Although aspirin strongly inhibited arachidonate-induced platelet aggregation in vitro, the drug offered little protection against arachidonate-induced stroke.