Local cerebral blood flow(1-CBF) and somatosensory evoked potentials(SEP's) were measured during focal cerebral ischemia and after restoration of cerebral blood flow in 17 mongrel dogs. The animals were divided into 3 groups in order to examine changes in SEP's following cerebral ischemia of varying duration and degree. The first portion of the middle cerebral artery(M1) and the common trunk of the anterior cerebral artery (A2) were occluded using a transorbital approach with an operating microscope. In group A(n=7), both Ml and A2 were occluded for two hours. In group B(n=6), M1 was occluded for four hours and A2 was occluded for two hours beginning one hour after the M1 occlusion. In group C(n=4), M1 was occluded for two hours and A2 was occluded for one hour beginning 30 minutes after Ml occlusion. SEP's were suppressed following the decrease of 1-CBF and disappeared below 20ml/100g/min. Following restoration of flow, the SEP's of group B (where the longest and most severe degree of ischemia was produced) recovered poorly or not at all. In animals where SEP's failed to recover well, marked hyperemia and brain swelling were frequently found following restoration of flow. In group C where the duration and degree of ischemia were less, the SEP's improved better, and hyperemia and brain swelling were not found.In this study, two major factors may be considered to have influenced the recovery of SEP' s following restoration of flow. The first factor is the duration and the degree of ischemia, and the second is hyperemia and brain swelling. In animals where the degree of ischemia was severe, SEP's failed to recover well, and marked hyperemia was found in spite of graded restoration of flow. It is felt that the restoration of flow may actually be harmful if collateral circulation is poor and/or duration of ischemia is long.