Lactography is a novel technique that allows the continuous on-line registration of brain extracellular lactate in the freely behaving animal and that is based on a fluorimetric enzymatic assay of brain dialysates. Electroconvulsive shock, activation of the glutamate receptor (NMDA-type) and mild stress (immobilization, cold stress or handling) result in transient increases in the efflux of lactate from the rat hippocampus. The increase following immobilization stress was attenuated by the NMDA-receptor antagonist 2-amino-5-phosphopentanoic acid and after several pre-exposures to this stressor. These experiments suggest that mild stress activates glutamatergic neurons, which may be less after habituation to stress.