1. We have used n.m.r. spectroscopy to measure rubidium concentrations in the skeletal muscle of live intact rats. Using a 1.9 T superconducting magnet and an ear-phone coil tuned to both protons (1H) and rubidium (87Rb), it was possible to make measurements of both tissue rubidium content and water content, and from these measurements to obtain the rubidium concentration.

2. The n.m.r. estimate of rubidium concentration in muscle in vivo was found to be a constant 31% (sem 4%) of that estimated by flame atomic absorption spectroscopy in an extract of excised muscle. This is close to the predicted theoretical n.m.r. visibility of 33%. The visibility was constant for muscle rubidium concentrations ranging between 10 and 34 mmol/l.

3. Rubidium concentration measurement by this method is unaffected by variations in sample geometry, sample volume, tissue conductivity, coil tuning and amplifier gain.

4. By using this method to measure changes in tissue rubidium concentration with time in the same animal, it should now be possible to assess the activity of ion transport systems, such as sodium- and potassium-activated adenosine triphosphatase in vivo, by measuring the rates of change of tissue rubidium concentrations during the administration of rubidium salts.

5. This method could also be used to measure the absolute concentration of any n.m.r.-visible nucleus and could be applied to man.

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