On the internally perfused, voltage-clamped squid giant axon, the effect of pH on the potency of tetrodotoxin in blocking the sodium current has been re-examined at pH 7.8 and 8.8. Confirming previous studies, tetrodotoxin is weaker at pH 8.8, when the deprotonated C-10 -OH makes the toxin molecule a zwitterion. In contrast to previous studies, our results, based on full dose--response relations, show that the relative potency at the two pH values appreciably exceeds the ratio of the abundance of the protonated C-10 form. In a medium of lowered ionic strength, tetrodotoxin is weaker, and the relative potency at pH 7.8 and 8.8 approaches the ratio of the relative abundance of the C-10 -OH. The results are believed to support hydrogen bonding at C-10 as a contributing factor in the binding of tetrodotoxin to the membrane receptor.