1H- and 31P-magnetic resonance spectroscopy was used to monitor intracellular lactate, phosphorus metabolites and pH in superfused brain slices from 2- to 9-day-old rats. N-Methyl-D-aspartate (NMDA) (100 microM, 0.5-3 min) was applied in the extracellular magnesium-free perfusion medium. NMDA induced intracellular metabolic acidosis, i.e., an increase of freely mobile lactate levels and an 0.3 pH unit acidification. This was abolished when the extracellular glucose supply was reduced. Experiments also indicate that acidosis is not responsible for the cell damage resulting from activation of NMDA receptors in hypoglycemic conditions.