Recently, the authors established an animal model of fatigue. The fatigued animals showed reduced 2-[18F]fluoro-2-deoxy-D-glucose uptake in their brain, although their blood glucose level did not differ from that of the control animals. For further clarification, the study measured regional cerebral blood flow, ATP level, and the ability of mitochondria to produce ATP in the brain of the fatigued and control rats. The fatigued animals showed almost equal regional cerebral blood flow, a significantly higher ATP level, and almost equal mitochondria ability to produce ATP. These data suggest that decreased energy utilization in the brain is a feature of fatigue.