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1. The intracellular pH of intact single muscle fibres of the giant barnacle was measured directly with a glass micro-electrode following prolonged (2-5 hr) equilibration in one of three solutions: normal Ringer, CO2 Ringer and NH4+ Ringer. 2. The intracellular pH of identically-prepared fibres from the same specimen was measured indirectly from the distribution of DMO following prolonged equilibration in the same solutions. 3. The DMO-pH compared favourably with the electrode-pHi provided DMO-pHi was calculated from the values of the indicator compounds, [14C]DMO and [3H]inulin, obtained by extrapolating the slow uptake phase to time zero. 4. Following prolonged equilibration, the transmembrane H+ ion distribution was found to vary with the membrane potential but not in accordance with a simple Gibbs-Donnan equilibrium. 5. A model which recognizes the existence of two independent net fluxes for H+ across the membrane in developed to explain the results. One of the fluxes represents passive diffusion and the other represents the so called H+-pump. The model predicts the H+-pump rate increases by two orders of magnitude when pHi is reduced from 7-2 to 6-7.