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1. Conflicting values of intracellular pH (pH_i) were obtained by Caldwell in crab muscle and by Carter et al. in rat muscle. To investigate this discrepancy, double and single barrelled pH sensitive micro-electrodes as described by Carter et al. were used both in rat muscle (in vivo and in vitro) and in large single fibres of crab (in vitro).

2. The pH_i values obtained in the superficial layers of muscle cells in rat and pH_i values obtained in the superficial area of large single fibres in crab seemed in agreement with the Donnan equilibrium. Furthermore, when the membrane potential (E_m) value of extracellular pH (pH_e) value was changed, each new pH_i value was obtained instantly.

3. The pH_i values obtained in deep layers of muscle cells in rat (pH_i 6·68) and in a deep area of large single fibres in crab (pH_i 6·96) did not agree with Donnan equilibrium; these pH_i values were little influenced by modifications in E_m or pH_e values.

4. These findings could be easily explained if the length of the sensitive portion of the pH micro-electrode exceeds the diameter of rat muscle cells and is smaller than the diameter of crab large fibres. The pH_i values obtained superficially in the two preparations and which seem in agreement with the Donnan equilibrium are artificially low because the E_m values recorded by the pH micro-electrodes are partly shunted.