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The internal pH of crab muscle fibres was measured using recessed-tip pH-sensitive micro-electrodes. Immediately following electrode penetration the mean internal pH was 7-21 +/- 0-02 (S.E. of mean) and the mean membrane potential was -64-9 +/- 0-6 mV (S.E. of mean). If H+ ions were passively distributed across the fibre membrane the internal pH would have been 6-39. 2. The internal pH tended to rise before stabilizing at a mean value of 7-27 +/- 0-02 (S.E. of mean). The difference between immediate and stabilized values is highly significant and suggests acid injury on electrode penetration. 3. Changing the membrane potential or external pH had only small, slow effects on internal pH. 4. External CO2 caused a large and rapid decrease in internal pH. With low concentrations of CO2, the effect was dependent on the initial pH as predicted by the Law of Mass Action. During a long exposure to 2-65% CO2 at pH 7-5, the internal pH returned slowly to its previous value, suggesting active transport of H+ (or OH- or HCO3-) ions across the fibre membrane. 5. The internal buffering power calculated from the response to 2-65% CO2 was 47-3 +/- 2-8 slykes (m-equiv H+/pH unit per l.) (S.E. of mean).