We have employed concurrent 31P- and 23Na-nuclear magnetic resonance (NMR) spectroscopy in conjunction with the paramagnetic shift reagents dysprosium-chelated tripolyphosphate and triethylenetetramine-hexa-acetic acid to observe the intracellular sodium and phosphorus signals in rat leg muscle. With induced ischemia in the leg, we find slowly falling phosphorylation potential. At a critical value of, associated with energetic failure of the Na+-K+ antiport, the intracellular sodium signal begins to increase. We find the following critical values: log, 3.12 +/- 0.32; pH, 6.86 +/- 0.13; Na+ influx with and without ouabain, 5.1 +/- 4.3 and 4.0 +/- 1.3 mol.l-1.h-1, respectively.