1. The potassium status of uraemic patients was assessed by measurements of leucocyte potassium, erythrocyte potassium, exchangeable potassium (42K) and total body water (3H). The difficulties involved in assessment of exchangeable potassium and whole body potassium are discussed, and the advantages of the leucocyte as a model for the study of intracellular potassium are described. 2. in twenty-three measurements in sixteen undialysed uraemic patients, two patterns of reduction in intracellular potassium concentrations could be discerned in the leucocyte: (a) due to reduction in potassium related to cell solids; (b) due to an increase in cell water. Both abnormalities were present in some patients. 3. Similar measurements were made in eighteen uraemic patients receiving regular haemodialysis against dialysis fluid with a potassium concentration of 1 mmol/l. in this group leucocyte potassium did not differ significantly from normal, while leucocyte water content was slightly less than normal. 4. It is concluded that disturbances of leucocyte water and potassium are common in advanced uraemia, and that these functions tend towards normal during regular dialysis using dialysis fluid with a low potassium content.