The behavior of the same ion-selective electrode (ISE) under three measurement modes: potentiometric, chronoamperometric and coulometric is compared using valinomycin-based K+-ISE as model system. The ISE response to decrease and to increase of the K+ ion activity is studied systematically. It is shown that the theoretical description of the time dependencies of current and of charge must take the concentration polarization in consideration, and the respective equations are presented. The concentration of K+ ion in blood serum samples is measured in the potentiometric, in the chronoamperometric and in the coulometric mode, and satisfactory results are obtained in all three cases.