KROGH, in his Croonian Lecture1 before the Royal Society, considered the apparent impermeability of striated muscle to sodium to be due to an active extrusion of sodium ions. Discussing this view, Conway2 calculated the minimal energy required for extrusion of sodium from the normal frog's sartorius if sodium entered as fast as potassium. The result was that more than the energy available from the metabolism of the resting muscle would be needed.