Localized proton NMR spectroscopy was used to study cerebral metabolism in the visual cortex of healthy adults during rest and photic stimulation. Basal lactate levels showed considerable interindividual differences ranging from below detectability (mM) to about 1 mM without consistent alterations during photic stimulation. Local brain glucose levels were significantly reduced (≈ 50%) during the entire period of photic stimulation and recovered to resting levels (≈0.8 mM)within 10 minafter the end of stimulation. This decrease reflects the establishment of a new equilibrium due to enhanced delivery (blood flow) and enhanced consumption. The absence of lactate accumulation supportsthe hypothesis of a rapid efflux of lactate from brain tissue under activated conditions.