Cerebral lactate, pyruvate and adenosine triphosphate (ATP), and arterial acid-base balance were measured in normotensive rats (NTR) and spontaneously hypertensive rats (SHR) at various time intervals following bilateral carotid occlusion. In SHR, a great and progressive increase in cerebral lactate and lactate/pyruvate ratio (L/P) ratio) during the first 5-6 hours after occlusion was followed by a gradual decrease as the time interval increased. Cerebral ATP in SHR decreased as cerebral lactate increased, and an inverse relationship was observed between lactate and ATP. On the other hand, in NTR, there were minimal changes in cerebral lactate and L/P ratio following carotid occlusion. These results suggest that bilateral carotid occlusion might cause severe ischemic damages of the brain in SHR, but not in NTR. The hemodynamic factor as a cause of cerebral ischemia in hypertensives is discussed.