1. A study has been made of the interaction between Na+ and K+ on the adenosine triphosphatase activity of erythrocyte ‘ghosts’, and on the K+ influx and Na+ efflux of intact erythrocytes. The adenosine triphosphatase activity and the ion movements were greater at a low external K+ concentration in the absence of Na+ than they were in the presence of 150mm-Na+. The inhibition by external Na+ of K+ influx had an inhibitory constant of 5–10mm. 2. Activation by K+ of kidney microsomal adenosine triphosphatase was retarded by Na+, and activation by Na+ was retarded by K+. Fragmented erythrocyte membranes behaved similarly. 3. These observations suggest that there is competition between Na+ and K+ at the K+-sensitive site of the membrane.
