Electrophysiological studies are being done to investigate the mechanism of ion translocation in epithelia. Studies of the isolated perfused rabbit cortical collecting tubule are aimed: 1) at the mechanisms of H ion secretion and the origin of the luminal positive voltage consequent to inhibition of active transepithelial Na transport either by ouabain or incubation of tubules in the total absence of Na and K, and 2) the determination of the chronic effects of DOCA pretreatment of the rabbits on their acidification process. The isolated frog skin is being studied with intracellular microelectrode techniques in both intact skins and isolated epithelia permitting cellular penetrations from both apical and basolateral surfaces of the tissue. Particular emphasis will be on 1) studies with K and Cl intracellular ion selective microelectrodes under various conditions of transepithelial Na transport especially with regard to the mechanisms of the Na ion K-ATPase and the K selectivity of the basolateral barrier, and 2) on the effects of a cellular acidosis created by elevation of CO2 tension of the bathing solution. Additionally, the Na rectification process at the apical barrier of the cells will be studied under a variety of conditions known either to simulate or inhibit Na entry at this barrier.