1. The construction and properties of a new design of pH-sensitive micro-electrode are described. The electrodes are very durable, and have a recessed configuration so that only the extreme tip, which can be as small as 1 μm in diameter, needs to enter the cell.2. The average intracellular pH in thirty-two snail neurones was 7·4. This was not in accord with a passive distribution of H+ ions across the cell membrane.3. Changing membrane potential or external pH had only slow effects on internal pH.4. Removing external K had no effect, and removing external Na had only slow and variable effects on intracellular pH.5. Anoxia, azide and DNP all caused a slow fall in internal pH.6. External CO2 caused large and rapid decreases in internal pH, which external bicarbonate appeared to offset slowly. Injected bicarbonate increased internal pH.7. The size of the pH changes caused by CO2 suggested a minimum intracellular buffering power of 25 m-equiv H+/unit pH per l., equivalent to that of 150 mM Tris maleate, pH 7·4.8. External ammonia caused a large and rapid increase in internal pH, while the injection of ammonium ions had the opposite effect.
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