1. The patch clamp technique was used to study the effects of internal and external pH on the Ca(2+)‐ and voltage‐activated maxi K+ channel present in the apical membrane of Necturus gall‐bladder epithelial cells. 2. When the pH of the solution bathing the cytosolic side of inside‐out patches (pHi) was lowered from 7.9 to 6.9, with internal free Ca2+ concentration ([Ca2+]i) buffered below saturation levels for the channel gating sites, channel open probability (Po) decreased. At saturating Ca2+ concentrations, Po was near 1.0, and unaffected by pHi. The results are consistent with a competitive interaction between Ca2+ and H+ at regulatory binding sites. Kinetic analysis assuming competitive binding yields a Hill coefficient for H+ of 1.3. 3. At sub‐maximal [Ca2+]i, changing the pH of the solution bathing the extracellular surface of the patch (pHo) between 8 and 7, had no effect on maxi K+ channel Po, but lowering pHo to 6 or 5 significantly reduced Po. At saturating [Ca2+]i, Po was independent of pHo. 4. There were no effects of either pHi or pHo on single‐channel conductance. 5. Inasmuch as reductions in either pHo or pHi decrease maxi K+ channel Po, changes in maxi K+ channel activity account in part for the reduction of apical membrane K+ conductance elicited by acidification of the bathing medium. The dominant effect of pH on maxi K+ channels is on the cytosolic surface of the membrane. 6. The change in Po elicited by small changes in [H+]i (delta Po/delta [H+]i) is ‐7.6 microM‐1, compared to delta Po/delta [Ca2+]i = 2.6 microM‐1, both at Vm = ‐30 mV and at physiological intracellular [H+] and [Ca2+]. This implies that [H+]i and [Ca2+]i have opposite effects on channel Po at physiological levels and underlines the importance of pHi in channel gating.