1. The effect of extracellular pH changes on the voltage-current relation of frog muscle membrane has been studied using intracellular microelectrodes. To reduce the cation conductance of the membrane, potassium in Ringer solution was replaced by rubidium.2. When the relatively impermeant methyl sulphate ion replaced extracellular chloride the membrane conductance was low, time independent and little influenced by extracellular pH changes. The voltage-current relation was linear at both high and low pH values.3. In rubidium Ringer solution at pH 7.4 the membrane conductance fell as the inside of the fibre was made more negative, in a manner consistent with the predictions of the constant field theory.4. At a high pH value (9.8) the resting conductance was high, but it fell steeply as the membrane potential was increased; for large hyperpolarizing voltages the membrane current tended to a limiting value. The voltage-current relation crossed those recorded at pH 7.4 and 5.0.5. In acid Ringer solution (pH 5.0) the resting membrane conductance was low and remained constant until the membrane potential was hyperpolarized more than 30 mV beyond the resting value; the conductance then rose as the membrane potential was further increased. For large hyperpolarizations the membrane conductance was higher at pH 5.0 than at pH 9.8.6. Experiments using two successive, identical, constant current pulses suggested that the membrane conductance altered during the passage of current across the membrane; in alkaline solution the conductance fell with time, in acid solution it rose.7. Because no time or voltage dependence of the membrane conductance was seen in the absence of chloride ions it is inferred that the movements of chloride ions across the muscle membrane are responsible for both the time and voltage dependent alterations in membrane conductance seen at different pH values.