1. Erythrocyte sodium concentration and fluxes were measured in patients with acid-base disturbances, hypokalaemia and hyponatraemia. Results were similar to those obtained with normal erythrocytes exposed to artificial in vitro alterations.
2. Erythrocyte sodium content and influx varied directly with extracellular bicarbonate which appeared to influence membrane permeability.
3. Hypokalaemia increased the erythrocyte sodium content by decreasing active transport initially. When a new high erythrocyte steady-state sodium concentration was reached, active transport returned to normal but efflux and influx were increased considerably by the appearance of a large component of exchange diffusion in the hypokalaemic environment.
4. Hyponatraemia induced a decrease in sodium influx secondary to the decreased transmembrane sodium concentration gradient. A decrease in erythrocyte sodium content then ensued.
5. The results are discussed in relation to the assessment of cell membrane function in disease states.