1. Sodium and potassium transport rates were studied in normal human leucocytes exposed to iso-osmolal and hypo-osmolal extracellular fluid.
2. Hypo-osmolality of the extracellular fluid led to an increase in sodium influx and a decrease in potassium influx expressed as mmol h−1 kg−1 cell dry weight. The fall in potassium influx was smaller than the rise in sodium influx and was confined to the ouabain-insensitive portion of the flux.
3. The rate constants for sodium and potassium efflux did not differ significantly between the iso-osmolal and hypo-osmolal media.