Platelet deposition in the microcirculation may play a role in focal cerebral ischemia. We investigated platelet deposition in selected parts of the cat brain after temporary middle cerebral artery occlusion. Ten anesthesized cats were given autologous indium-lll-labeled platelets and chromium-51-labeled erythrocytes. The right middle cerebral artery was occluded with miniature aneurysm clips for 3 hours via a transorbital approach; blood pressure was reduced concomitantly to decrease the collateral circulation. Removal of the clips initiated a 45-minute period of normotensive reperfusion. After sacrifice, the brain was removed and sectioned for comparison of right-versus left-hemisphere platelet deposition. Platelets were selectively deposited in the territory of the occluded right middle cerebral artery. Significant deposition was found in the caudate nucleus, internal capsule, parietal cortex, and the centrum semiovale. Our findings support the evidence that platelets are deposited in the microvasculature during temporary severe focal cerebral ischemia. (Stroke 1989;20:664-667) P latelet deposition in the cerebral microcirculation may play a role in focal cerebral ischemia. Morphologic techniques failed to implicate erythrocytes in microcirculatory obstruction during focal cerebral ischemia.1 Few studies have focused on the role of platelets in microcirculatory changes during cerebral infarction.2 -5 No study has used an experimental model analogous to clinical focal cerebral ischemia.We undertook this study to evaluate platelet deposition induced by temporary occlusion of the right middle cerebral artery (MCA) followed by reperfusion in cats. Materials and MethodsBlood (30 ml) from adult cats was collected through a catheter inserted into the femoral vein with citrate-phosphate-dextrose (CP-NIH formulation) as the anticoagulant. Lactated Ringer's solution (20 ml) was administered intravenously to augment vascular volume. The blood was treated with 50 nmol prostaglandin E] to prevent platelet activation and was centrifuged at 280g for 20 minutes at 25° C to separate platelet-rich plasma from the erythrocytes. The platelets were pelleted by centrifugation at l,500g for 15 minutes, and the From the Departments of Neurosurgery (J.J.J., R.M., R.M.C.)