Neuronal activation demands metabolic energy that must be supplied within seconds. The study of the mechanisms catering for such demand has been hampered by lack of techniques capable of measuring metabolic flux with the required resolution. A Fluorescence Resonance Energy Transfer (FRET) – based microscopy method is presented here which offers much improved spatiotemporal resolution over existing methods like 2-deoxyglucose autoradiography and NMR spectroscopy. Applied to mouse astrocytes in culture and in brain slices, the method showed that glycolysis can be activated within seconds by physiological concentrations of extracellular K⁺ . No activation was observed in response to glutamate, widely regarded as responsible for neuronal modulation of astrocytic metabolism. The effect of K⁺ on glucose metabolism was readily reversible, required Na⁺/K⁺ ATPase pump activation and was mediated by depolarization-induced alkalinization (DIA), a phenomenon of hitherto unknown physiological significance. A rapid activation of astrocytic glycolysis was also observed in brain slices exposed to exogenous K⁺ and during electrical stimulation. These results expose a novel phenomenon linking synaptic activity to timely local production of lactate by astrocytes and ascribe astrocytic pH a central role in neurometabolic coupling and neurovascular coupling. This technique to measure glycolysis in single cells, in real time and reversibly, may also be used to investigate the transport and metabolism of glucose in other cell types.