Active sodium transport (outflux or efflux) in red blood cells generally has been measured by assessing the amount of outflux inhibited by digitalis glycosides (outflux-fraction I). The presence of a ouabain-uninhibited sodium outflux (outflux-fraction II) attributable either to a second active transport mechanism or to exchange diffusion has been the subject of recent investigations. In the present study a variety of transport inhibitors, including ouabain, ethacrynic acid, furosemide, oligomycin, and amiloride, were studied for their effects on these components of sodium transport in RBC. In the presence of ouabain both ethacrynic acid and furosemide exerted similar effects on sodium outflux, inhibiting approximately 0.5 mmoles/L of cells per hr. This component of sodium outflux has been called outflux-fraction II. Ethacrynic acid showed no inhibitory potency when ouabain and furosemide were present, thereby suggesting that the same outflux component (fraction II) was affected by ethacrynic acid and by furosemide. In addition, furosemide reduced sodium influx to the same extent that it reduced sodium outflux. Outflux-fraction II, as defined by furosemide, did not contribute a net sodium outflux. These results of sodium outflux and influx experiments confirm the existence of a transport pathway which does not contribute to net flux and which fits the definition of exchange diffusion. The inhibitory effect of furosemide on outflux-fraction II remained despite the use of a sulfhydryl protective reagent, whereas the effect of ethacrynic acid was obliterated. No combination of inhibitors was found which affected the residual or uninhibited sodium outflux (0.4-0.5 mmoles/liter of cells per hr). Oligomycin possessed an inhibitory potency less than that of ouabain, and it exerted no effect on sodium outflux if it was superimposed upon ouabain inhibition. Amiloride proved to be a very weak inhibitor of sodium outflux in human erythrocytes.