The effects of low intracellular pH (pHi) on the membrane currents of snail neurone somata were studied using the internal perfusion and ion-sensitive micro-electrode techniques. Recordings with pH-sensitive micro-electrodes made while the pH of the perfusion solution was changed between 7.3 and 6.3 indicated that only with high buffer concentrations (100 mM) could pHi be changed effectively. H+ was slower to exchange into the cytoplasm than an unbuffered ion such as K+. When pHi was decreased to 5.9, large outward H+ currents could be recorded at voltages positive to -30 mV. The time course and amplitude of these currents were such that they did not affect the measurement of the peak amplitude of the fast transient K+ current (A-current), but severely contaminated both Ca2+ and delayed K+ current measurements. Low pHi blocked the A-current. The titration curve was consistent with the binding of two H ions to a site with a pK of 6.05 to block the channel. Low pHi appeared to block the slow inactivation of the delayed outward current without greatly changing its peak amplitude. However, when correction was made for the increase of H+ current at low pHi, the effect of internal H+ was found to be a block of the delayed K+ current with no consistent effect on inactivation. The Ca2+ current was also decreased at low pHi, but we were unable to determine whether this was a direct effect of pHi or secondary to a rise in internal free [Ca2+]. If no correction was made for H+ currents, the block of the Ca2+ current appeared greater and more reversible than it actually was. We conclude that under certain conditions, such as low pHi, the H+ current is a significant fraction of the total outward current in snail neurones, and may also be in a variety of other cells. The H+ currents must be accounted for under such conditions in order to study accurately the properties of K+ and Ca2+ currents.