The amplitude and rate of activation of the voltage-dependent H(+) pathway in intact Helix neurones were investigated using standard two-electrode voltage-clamp techniques. Na(+) and K(+) currents were inhibited by a Na+-free, tetraethylammonium(TEA(+)) (low-Cl(-)) saline and by use of Cs(+)-filled electrodes. Ca2(+)currents were abolished by holding the membrane at --15 to --10 mV. Depolarizing voltage pulses from these low potentials activated outward currents whose tail current reversal potential shifted with changes in intracellular and extracellular pH, but not with changes in external KC1; thus these remaining currents are carried by hydrogen ions. Furthermore, the amplitude of the voltage-dependent outward current increased as the outward gradient for H(+) was increased and a rise in pHi shifted the activation towards negative potentials. At physiological pH levels, H(+) currents were typically 60 nA at 30 mV (cell diameter 200-250 -μm). H(+) currents were rapidly activated; the time to half maximal current at 30 mV was less than 5 ms in the pHi range tested (7-4-6-9) (pHe 7-4). The H(+) pathway will therefore be activated by individual action potentials and may play an important role in pH homeostasis during intense neural activity.