Recent advances in analytical electrochemistry have provided highly selective potassium ion exchangers and robust polymeric membranes that can be used to measure the potassium activity in plasma or whole blood. The relationship between the activity measurement and the more familiar concentration measurement is explored. The ion selective electrode (ISE) has certain practical advantages and these have been studied in relation to their use in an intensive care unit. The application of these membranes to the continuous measurement of potassium in the circulation is discussed.