The influence of changes in ionic composition of the bathing solutions on intracellular electrical potentials in frog skin has been examined. When the skin bathed in SO4 Ringer's solution is penetrated with a microelectrode two approximately equal potential jumps were frequently observed and most experiments were carried out with the electrode located between these steps. Substitution of Cl for SO4 in the bathing solutions caused a decrease in PD across both the "outer" and "inner" barriers. When the skin was short-circuited an average intracellular potential of -18 mv was found with both Cl and SO4 Ringer's. With the skin in SO4 Ringer's, decrease in Na concentration of the outside solution caused a decrease in PD between the microelectrode and the outside solution which was approximately the same as the decrease in total skin PD. With SO4 Ringer's, an increase in K concentration in the inside solution caused a marked decrease in total skin PD. However, only 50 per cent of this change occurred at the inner barrier, between the microelectrode and the inside solution. The remainder of the change occurred at the outer barrier. This observation does not appear to be consistent with the model of the skin proposed by Koefoed-Johnson and Ussing (Acta Physiol. Scand., 1958, 42, 298).

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