1. Sodium and potassium transport rates in human leucocytes were measured in vitro at different external potassium concentrations.
2. At nominally zero external potassium concentrations, the ouabain-sensitive sodium efflux was reduced to less than 20% of its maximum value. There was evidence that under these conditions a ouabain-sensitive sodium-sodium exchange occurs.
3. Both total and ouabain-insensitive potassium influx increased with increasing external potassium concentration. The ouabain-sensitive potassium influx showed saturation.
4. Ouabain-insensitive potassium efflux was also stimulated by increasing the external potassium concentration, suggesting significant potassium-potassium exchange at physiological external potassium concentrations.