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1. A study has been made of the ionic basis of the smooth muscle membrane potential by changing the ionic environment of the ureter and recording the resultant potential changes with the sucrose gap. The relation of these potential changes to the membrane potential of individual cells has also been studied.

2. It is shown that if the ionic environment of all the cells in the tissue is uniformly changed, then the potential change recorded by the sucrose gap is proportional to the magnitude of the short circuit factor, the liquid junction potential change at the sucrose-test solution interface, and the amplitude of the membrane potential change of a single cell.

3. A simple method is described for determining the steady value of the short circuiting factor, and the liquid junction potential changes, so that the recorded potentials can be corrected to give membrane potentials.

4. The action of isotonic potassium sulphate and isotonic potassium chloride on the membrane potentials of the ureter smooth muscle cells is described.

5. When the extracellular potassium concentration is changed reciprocally with the extracellular chloride concentration in order to maintain these ions in a Donnan equilibrium across the muscle cell membrane, the membrane potential is found to decrease by 53 mV for a 10-fold change in external potassium concentration, for concentrations above 10 mM.

6. It is concluded that above an external potassium concentration of 10 mM the membrane potential of ureteral smooth muscle cells obeys the prediction of the Nernst equation for a potassium electrode.