The diffusional fluxes of urea, potassium and bicarbonate across the dialytic membrane (external balance), and across the cellular membrane (internal balance), were determined in 7 patients in haemodialysis using potassium free dialysate and dialysate containing 2.0 mEq/1 of potassium. The results obtained show an inverse correlation between extraction of potassium and intake of bicarbonate in both external and internal balance. This is probably due to the increase in membrane electrical potential resulting from a fall in blood potassium and emphasizes the importance of electrical driving forces in diffusional fluxes across cellular membranes.