A method has been developed to measure the rates of glucose utilization in the individual structural and functional components of the central nervous system. It can be applied to conscious as well as anaesthetized animals. The method is based on the use of [14C]deoxyglucose as a tracer for glucose consumption. [14C]Deoxyglucose-6-phosphate accumulates in the tissue in a mathematically definable relationship to the rate of the tissue's glucose utilization. The [14C]deoxyglucose-6-phosphate concentrations in the various tissues of the nervous system are measured by a quantitative autoradiographic technique. The autoradiographs themselves are pictorial representations of the relative rates of glucose consumption in these tissues. Application of this method to rats and monkeys in various states of altered function demonstrates a clear and close relationship between the local levels of functional activity and energy metabolism. The method appears to be useful for mapping functional neural pathways on the basis of evoked metabolic responses (Plum, Gjedde and Samson, 1976).