Serial measurements of regional cerebral blood flow were made by the 133Xe inhalation method during the early stages of sleep and wakefulness in eight normal volunteers and 12 patients with narcolepsy. Electroencephalogram, electro-oculogram, and submental electromyogram were recorded simultaneously. In normals, mean hemispheric gray matter blood flow (Fg) during stages I and II sleep was significantly less (−9.2 percent) than waking values (84.3 ± 13 ml per 100 gm brain per minute). Maximum regional blood flow decreases during sleep occurred in the brainstem-cerebellar (−25.1 percent), right inferior temporal (−23.1 percent) and bilateral frontal (−18.9 percent) regions (p < 0.05). In patients with narcolepsy, mean hemispheric Fg while awake was 80.5 ± 13 ml per 100 gm brain per minute. During REM sleep (n =2), mean hemispheric Fg increased by 9.8 percent concurrently with large increases (+34.6 percent) in brainstem-cerebellar region flow. During stages I and II sleep without REM (n=6), there were significant increases in mean hemispheric Fg of ±20.2 percent (p < 0.01) and brainstem-cerebellar Fg of 38.0 percent (p < 0.01), just the opposite of changes in normals. In narcolepsy, there appears to be a reversal of normal cerebral deactivation patterns, particularly involving the brainstem, during stages I and II sleep.