The somatosensory evoked potential (SEP) measured in response to median nerve stimulation was correlated with cortical and white matter cerebral blood flow (CBF), adenosine triphosphate (ATP), and lactate levels in 14 cats subjected to graded hemorrhagic hypotension following bilateral carotid artery ligation. Three additional cats served as controls. Regional CBF was determined by the hydrogen clearance method, and the time for conduction of the sensory stimulus from the thalamus to the cortex (the thalamocortical conduction time), was used to assess SEP latency changes. A reproducible sequence of changes occurred in the SEP as ischemia developed. There was an early conduction delay that correlated well with mild white matter ischemia. Amplitude reductions in the SEP began as significant cortical ischemia occurred. The cortical SEP was abolished when white matter CBF and ATP fell to critical levels.