Patients requiring hemodialysis for end-stage renal disease are vulnerable to wide fluctuations in plasma potassium concentration [K+]p. Dialysis of K+ can result in rapid and large changes of [K+]p and result in significant morbidity. Since blood is routinely diverted extracorporeally to the dialyzer, the measurement of [K+]p could be made 'on-line' in this circuit with an ion-selective electrode (ISE) with negligible additional risk. Valinomycin/tetraphenylborate embedded in polyvinylchloride (PVC) electrodes of high K:Na selectivity were manufactured in our laboratory. The electrodes were calibrated in vitro and placed in a T diverter tube upstream from the dialyzer. Intermittently small amounts of blood were diverted to flow past the electrode allowing measurement of the electrical potential developed between the ISE half cell and a Ag:AgCl electrode placed 4 mm from the valinomycin membrane which could be read as [K+]p ISE. Blood flowing through the T diverter was then collected in glass tubes for measurement of [K+]p by flame photometry. Forty-six comparative measurements were made for 6 patients who had given informed consent for study. Least squares regression analysis for all observations gave a correlation coefficient of 0.95 and a regression equation of y = 1.1 X -0.39. Only six samples were discrepant by 0.5 mmol/l, and all of these had [K+]p greater than 5.0 mmol/l where the ISE measurements were less precise. Although the correlation of the two techniques was less than perfect, the ISE shows potential utility for monitoring trends of [K+]p instantaneously during hemodialysis.