Local blood flow in the cortex and thalamus was measured by the hydrogen clearance method in spontaneously hypertensive rats (SHR) and normotensive rats (NTR) before and after bilateral carotid occlusion. There were no differences in the resting blood flow values between SHR and NTR. Following carotid occlusion cortical blood flow in SHR was markedly reduced to 17% of the resting level at 1 h and, further, to less than 10% at 3--5 h period, while in NTR it decreased only to 36--38% during 5 h occlusion. Thalamic blood flow in SHR was decreased to 39% at 1 hr and to below 20% at 3--5 h, while in NTR it remained approximately 40% of the resting level during 5 h occlusion. The blood flow reduction in either cortex or thalamus after carotid occlusion was much greater in SHR than in NTR. This difference was highly significant. The increased cerebral vascular resistance caused by persistent hypertension may play an important role in a greater reduction of blood flow in SHR after carotid occlusion. Relation of the blood flow reduction to the brain metabolism is discussed.