The Hodgkin-Huxley kinetic parameters, alpha h and beta h, which govern the rate of recovery from and development of sodium channel inactivation, respectively, have been measured as a function of membrane potential and external pH using a three-pulse protocol. alpha h but not beta h is substantially accelerated by reducing external pH from 7.4 to 6.4. The alpha h vs. voltage curve appears to be selectively shifted in the depolarizing direction by approximately 12 mV for this pH change, giving an apparent, h infinity curve shift of approximately 6 mV in the same direction (less inactivation).