Experiments were carried out to test the hypothesis that lactate reduces the neurotoxicity of glutamate in vivo. MAP2 immunohistochemistry was used to measure lesion size, and microdialysis to measure the changes in glucose and lactate in the extracellular compartment. After implantation of a microdialysis probe 100 mM glutamate with or without 6 mM lactate was added to the perfusion medium and infused into the cortex of unanesthetized rats. Infusion of 100 mM glutamate for a period of 30 min produced a lesion of 6.05 ± 0.64 mm3, an increase in lactate of 124 ± 19% above basal and a 21 ± 9% reduction of glucose below basal level. When 6mM L-lactate was perfused together with 100 mM glutamate there was a significant reduction in the size of the lesion and there was no reduction in dialysate glucose. When L-lactate was replaced with D-lactate the lesion size and the increase in dialysate lactate were greater than after glutamate alone. The neuroprotective role of L-lactate is attributed to its ability to meet the increased energy demands of neurones exposed to high concentrations of glutamate. © 2001 Wiley-Liss, Inc.