Both sodium and hydrogen ions are in lower concentration in muscle cell water than would be predicted from thermodynamic considerations alone, indicating active transport of these two ions out of muscle cells, To determine whether their transport is coupled, intact rat diaphragms were incubated at an external pH of 7.40, and sodium efflux was altered by exposure to ouabain. Despite a sixfold increase in intracellular sodium concentration, cell pH measured simultaneously by the distribution of both a weak acid (pH DMO) and a weak base (pH nicotine) was unaffected. Similarly, when sodium influx was decreased by incubating intact diaphragms in low sodium isoosmolar buffer, pH DMO and pH nicotine were unaltered. In addition, a pH-sensitive reaction, the evolution of 14CO2 from 1, 4-14C citrate by rat hemidiaphragm, was not changed by exposure to ouabain or low sodium media. These experiments demonstrate that in diaphragm muscle sodium and hydrogen ion transport appear not to be coupled, since alterations in sodium transport are unaccompanied by a change in cell pH. Although changes in membrane potential may explain part of the results, the data are most consistent with the hypothesis that active hydrogen ion transport in skeletal muscle in directly coupled to metabolic energy-producing reactions.