The recent introduction of miniature liquid ion-exchanger electrodes provides the possibility of overcoming previously encountered limitations on the measure-ment of specific ionic activities in biological fluids. Hydrogen ion and sodium ion activities have been stuccessfully determined by means of glass-membrane electrodes(l-5). However, in the presence of sodium, glass electrodes are usually inadequate for measuring potassium, divalent cation or anion activities(4). The adaptation of the liquid ion-exchanger principle for use with microelectrodes, permits measurement of K+ and Cl- activities on botlh sides of the cell mem-brane(6-8). We hlave constructed potassium-selective liquid ion-exchanger microelectrodes to measure K+ activity in rat distal tubules. Because this technique is still in a relatively early stage of development, it may be useful to review some details of the theoretical basis for the method and some of the practical considerations rele-vant to its application. Our experience has been with K+ ion-exchanger elec-trodes but the same teclhniques apply to anion-sensitive electrodes. Liquid ion exchangers are solutions in an organic solvent of a charged organic compound(9). Such ion-exchanger solutions were originally developed for indus-trial purposes; in the early 1960's they were increasingly studied as model mem-brane systems lhaving many of the properties of carrier-medicated ion permea-tion(9-11). A Ca selective macroelectrode was described by Ross(12), an
