Parallel measurements have been made of the oxygen consumption and efflux of radioactive sodium in pairs of frog sartorius muscles. Calculation of the amount of secretory work necessary for an active extrusion of sodium at the observed rate showed that it would involve the utilization of about one-tenth of the energy available from resting metabolism. This figure may reasonably be regarded as a lower limit to the efficiency of the secretory mechanism. Some of the measurements were made in a potassium-free Ringer's solution, and others with an external potassium concentration of 10 mM. In the potassium-rich medium, both the sodium efflux and the oxygen consumption were increased, the proportion of the energy production required for sodium extrusion remaining roughly constant. The action of dinitrophenol and other metabolic inhibitors on the sodium efflux in sartorius muscles was examined, but there were no very obvious effects.