1. A method is described that enables the calculation of resting membrane potential from the electrolyte and water contents in blood plasma and in a sample of human muscle tissue obtained by the percutaneous needle-biopsy technique. In this calculation, the previously described equations for calculating resting membrane potential via the intra- and extra-cellular distribution of chloride were combined with the equation utilizing potassium distribution over the cell membrane.
2. The method of calculation was applied to 60 healthy subjects divided into three groups aged 19–40, 41–60 and 61–85 years. The calculated resting membrane potential in the subjects as a whole was −88.4 mV (SD 1.35; n = 60). A lower value was observed in the group aged 61–85 years (−87.7 mV, SD 1.0; n = 12) than in the group aged 19–40 years (−88.6 mV; SD 1.4; n = 32). No difference was observed between female and male subjects.
3. The RMP calculated with the present method in 60 healthy subjects was also compared with previously published values in healthy subjects, measured by the Clarke electrode method, and with values calculated from electrolyte and water distribution measured by isotope-dilution techniques. The results obtained in healthy subjects with different techniques were very similar. Data were analysed from earlier published studies in experimental animals in which resting membrane potential ranged from −91 to −65 mV. The resting membrane potential calculated from electrolytes in plasma and muscle showed a very good agreement with resting membrane potential recorded directly.