Thirty-one cats were divided by age into 3 groups, young (Y), middle (M) and old (O). Continuous recordings of local cerebral blood volume (CBV) and frequent measurements of mean transit time of blood (t) were made from the Sylvian opercula after ischemia was produced by transorbital clipping of the middle cerebral artery at its origin (MCA occlusion). Control recordings were made simultaneously from the corresponding area of the contralateral cerebral hemisphere. MCA occlusion temporarily stopped cerebral blood flow (CBF) in the area supplied by the ipsilateral MCA, as indicated by a rapidly decreasing CBV and complete disappearance of hemodilution curves. Within 30 sec, CBF resumed with a dilatation of the vascular bed and reappearance of hemodilution curves through newly developed collateral channels. Despite a low CBF, below half the control, CBV recovered, overshooting the control level. The appearance of hyperemia in the ischemic area was statistically significant. Such "low perfusion hyperemia" was slower in appearance and of more diverse magnitude in group O than in group Y. This suggested that aging may lead to a decrease in rapidity of the vascular response to ischemia and impair the integrity of collateral vessels.