Ion-selective liquid ion-exchanger microelectrodes will be used to measure intracellular potassium and chloride activities in mammalian heart muscle. Initially, experiments will be confined to Purkinje fibers because they are easy to maintain in vitro, the cells are easy to penetrate and they display pacemaker activity. A recent study (Walker, unpublished) has shown that chloride is actively transported into frog heart muscle cells and preliminary experiments indicate that this is also the case in sheep and dog Purkinje cells. The properties of the chloride transport mechanism will be investigated using drugs, varying temperature and altering the ionic composition of the bathing solution. As intracellular chloride activity is altered during the course of these experiments, attempts will be made to relate those changes to observed changes in the membrane potential, including action and pacemkaer potentials. Potassium microelectrodes will be used to study the effects of cardiac clycosides on intracellular potassium activity in Purkinje fibers. Preliminary experiments indicate that ouabain, even at toxic concentrations that cause marked depolarization, does not reduce intracellular potassium activity. In fact it appears that at the proper concentration ouabain may cause a marked increase in intracellular potassium activity. Attempts will be made to fabricate a sodium glass microelectrode that will be suitable for recording from Purkinje cells so that intracellular sodium activities can be measured and correlated with the measurement of chloride and potassium.