The coupling between Ca²⁺ extrusion and H⁺ uptake by the ubiquitous plasma membrane calcium ATPase (PMCA) has not been measured in any neurone. I have investigated this with Ca²⁺‐ and pH‐sensitive microelectrodes in large voltage‐clamped snail neurones, which have no Na⁺–Ca²⁺ exchangers. The recovery of [Ca²⁺]_i and surface pH after a brief depolarization or Ca²⁺ injection was not slowed by hyperpolarization to −90 mV from a holding potential of −50 mV, consistent with a 1 Ca²⁺ : 2 H⁺ coupling ratio. Since Ca²⁺ injections proved difficult to quantify, and Ca²⁺ currents through channels were obscured by K⁺ currents, Ba²⁺ was used as a substitute. When the cell was bathed in Ca²⁺‐free Ba²⁺ Ringer solution, the K⁺ currents were blocked and large inward currents were revealed on depolarization. The Ca²⁺‐sensitive microelectrodes were sensitive to intracellular Ba²⁺ as well as Ca²⁺. With equal depolarizations Ba²⁺ entry appeared larger than Ca²⁺ entry and generated similar but slower pH changes. Ba²⁺ extrusion was insensitive to hyperpolarization, blocked by eosin or high pH, and about 5 times slower than Ca²⁺ extrusion. The ratio of the pH change caused by the extrusion of unit charge of Ba²⁺ influx to that caused by unit charge of H⁺ injection was 0.85 ± 0.08 (s.e.m., n= 8), corresponding to a Ba²⁺ : H⁺ ratio of 1 : 1.7. Both this ratio and the electroneutrality of the PMCA suggest that the Ca²⁺ : H⁺ ratio is 1 : 2, ensuring that after a Ca²⁺ influx [Ca²⁺]_i recovery is not influenced by the membrane potential and maximizes the conversion of Ca²⁺ influxes into possible pH signals.

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