1. The leucocyte has been used as a model for the study of intracellular sodium, potassium and water in uraemia. The advantages of this cell are described.
2. In undialysed patients with advanced renal failure leucocyte sodium and water contents were significantly greater than normal. Leucocyte potassium content (mmol/kg of dry solids) and concentration (mmol/l of cell water) were reduced.
3. In patients receiving regular dialysis leucocyte water was significantly reduced. Leucocyte potassium content was also reduced in this group, but leucocyte potassium concentration in cell water had returned to normal.
4. In the normal subjects and also in the dialysed patients leucocyte water correlated better with potassium than with sodium content. In contrast, in the undialysed uraemic patients leucocyte water correlated better with sodium than with potassium content, indicating that the increased cell sodium was an important determinant of the increased cell water in this group.