1. Potassium depletion was induced in rabbits with frusemide and measurements were made of plasma, leucocyte, erythrocyte and total exchangeable potassium before and during potassium depletion. 2. All measurements, except erythrocyte potassium expressed as mmol/kg of cells, decreased during the depletion period. 3. Significant correlations were observed between change in leucocyte potassium and change in total exchangeable potassium. 4. It appears that leucocyte potassium may prove a useful measurement of potassium depletion.