CNS tissue is well known to have large energy requirements. However, because of the difficulty of measuring rates of energy usage, relatively little is known about which cell types and which neurophysiological functions are the principal energy users. In experiments performed on rabbit retina in vitro, it was possible to measure O2 consumption and lactate production with the retinas under resting conditions and in different states of physiological activity. Resting energy consumption was large, as has been previously reported, and there were increases of up to 2.3 times with activity. Under some circumstances, the demands appeared to exceed the energy available. It was calculated that less than 5% of the energy generated by the retina was used for "vegetative metabolism" (i.e., for the anabolic reactions essential for viability), so that even in the resting state, the great majority of the energy usage appeared to be for function-related processes. This conclusion received further support from the finding that 50% of the energy generated was used for Na+ transport. The data obtained on retina are compared with published data on brain, which also suggest that a large fraction of the energy generated is used for function-related processes. It seems reasonable to conclude that by reversibly blocking the energy-demanding processes responsible for neurophysiological functioning, it would be possible to eliminate most of the energy requirements of CNS tissue and to reduce markedly its vulnerability to hypoperfusion.