We have used double quantum filtered (DQF) 23Na+ nuclear magnetic resonance (NMR) spectroscopy without shift reagents in order to monitor intracellular Na+ (Na+i) in a cortical brain slice preparation. The external Na+ (Na+o) signal was reduced by 95% by the DQF sequence compared with the directly observed 23Na+. The DQF 23Na+ signal is not exclusively due to Na+i, however, as 40% of this signal appears to arise from Na(+)-ions interacting with extracellular membrane proteins or proteins exposed at the cut surfaces of the slices. Veratridine increased instantly the DQF 23Na+ signal so that it reached 130.4 +/- 5.0% by 12 min. This shows that there was a significant contribution from Na+i in the DQF 23Na+ NMR spectra. Hypoxia of 30 min duration in the presence of 10 nM glucose did not influence intensity of the DQF 23Na+ signal. Aglycaemic hypoxia caused complete collapse of phosphocreatine (PCr) within 7 min whereas DQF 23Na+ first increased 15 min after the insult. This increase reached its maximal value of 125% after 25 min. There was an incomplete recovery of the DQF 23Na+ after aglycaemic hypoxia to 110% of the control value parallel to poor metabolic recovery. The presence of 10 mM extracellular Mg2+ had no apparent effect on the aglycaemic hypoxia-induced rise in Na+i indicating that it was linked to Ca2+ influx. Tetrodotoxin (TTx, 4.7 microM) did not influence the rise of Na+i caused by aglycaemic hypoxia. These results indicate that elevation of Na+i is a late consequence of energy failure in the cerebral cortex.