In frog skeletal muscle, the increase of intracellular pH (pHi) induced by insulin is correlated with an increase in intracellular Na+ when the sodium pump is inhibited by ouabain. Reversing the Na+ free energy gradient by substituting either Mg2+ or choline for extracellular Na+ converts the effect of insulin to a decrease in pHi, indicating that the action of insulin upon pHi is determined by the Na+ free energy gradient. Moreover, estimates of the Na+ free energy gradient indicate that both the direction and magnitude satisfy the hypothesis that this is the source of energy for the observed changes in pHi. Both the increase in intracellular pH induced by insulin and the associated increase in intracellular Na+ produced by this hormone in the presence of ouabain are blocked by amiloride. This drug also blocks the decrease in pHi by insulin when Mg2+ is substituted for Na+ in the Ringer. In Ringer containing Na+, the increase in pHi by insulin occurs when both metabolic and atmospheric sources of CO2 are eliminated by using a 100% N2 atmosphere. Thus, the mechanism stimulated by insulin is not a Na+-CO3(2-) cotransport system, but is either an Na:H exchange or a Na+-OH- cotransport system which can be inhibited by amiloride. The suggestion is advanced that the Na:H exchange mechanism is part of the membrane transduction system for insulin.