1. Exchangeable potassium, sulphate space and intracellular fluid have been determined in eight normal males and in seven uraemic male patients on a regime of twice weekly 8 h dialysis.
2. Intracellular water was more closely correlated with exchangeable potassium than were body weight or lean body mass.
3. There was no correlation between exchangeable potassium and plasma or erythrocyte potassium.
4. Uraemic patients had a 20% potassium depletion that may have been due to a restricted oral intake (40 mmol/day) and too little potassium (0–1 mmol/l) in the dialysis fluid.