Regional cerebral blood flow was measured with the intra-arterial 133 Xenon clearance technique using a multidetector device in 11 patients undergoing carotid angiography (with normal findings). During the flow studies the patients were awake and strict resting conditions were observed. The patients did not move or speak, and sensory stimulation were kept at a minimum. It was confirmed that the distribution of the grey matter blood flow showed a hyperfrontal pattern, the flow in frontal regions being significantly (20-40%) higher than in postcentral, occipital and temporal regions. There were no technical factors or morphological features of the telencephalon which could explain this difference. It was also shown that the distribution of the white matter flow and the relative weight of the grey matter corresponded in general to hemisphere morphology. Since in normal nervous tissue the blood flow is regulated by the neuronal activity, the following interpretation is given of the main finding. The hyperfrontal flow distribution of the grey matter (cortical) flow during resting wakefulness shows that there is a high activity in frontal "efferent" (motor-behavior) regions. At the same time there is a low activity in post-central and temporal "afferent" (sensory-gnostic) cortical areas. The high frontal activity suggests that in the resting conscious state--unaccompanied by movements, speech or behavioral reactions--the brain is active with an anticipatiory "simulation of behavior". The low postcentral flow, on the other hand, may possibly be related to a global inhibition of the sensory input. Several clinical as well as general biologic arguments are forwarded to support this interpretation. It is further pointed out that the hyperfrontal distribution of the resting activity in the cerebral cortex correlates to the resting EEG, in which lower frequencies (the alpha rhythm) predominate in postcentral and temporal regions where there is a low flow/activity, and high frequencies in frontal areas where the blood flow is high. This is in agreement with the finding that the blood flow and metabolism of the brain correlate to the EEG frequency content.