The voltage dependence of the voltage clamp responses of myelinated nerve fibers depends on the concentration of divalent cations and of hydrogen ions in the bathing medium. In general, increases of the [Ca], [Ni], or [H] increase the depolarization needed to elicit a given response of the nerve. An e-fold increase of the [Ca] produces the following shifts of the voltage dependence of the parameters in the Hodgkin-Huxley model: m(infinity), 8.7 mv; h(infinity), 6.5 mv; tau(n), 0.0 mv. The same increase of the [H], if done below pH 5.5, produces the following shifts: m(infinity), 13.5 mv; h(infinity), 13.5 mv; tau(n), 13.5 mv; and if done above pH 5.5: m(infinity), 1.3 mv; h(infinity), 1.3 mv; tau(n), 4.0 mv. The voltage shifts are proportional to the logarithm of the concentration of the divalent ions and of the hydrogen ion. The observed voltage shifts are interpreted as evidence for negative fixed charges near the sodium and potassium channels. The charged groups are assumed to comprise several types, of varying affinity for divalent and hydrogen ions. The charges near the sodium channels differ from those near the potassium channels. As the pH is lowered below pH 6, the maximum sodium conductance decreases quickly and reversibly in a manner that suggests that the protonation of an acidic group with a pK(a) of 5.2 blocks individual sodium channels.