The effects of pH changes on the miniature endplate current (mepc) and on endplate current fluctuations (acetylcholine [ACh] noise) were examined at the neuromuscular junction in vitro in two species of frogs. In Rana pipiens the relationship between the decay time constant of the mepc (τ') and pH had a symmetrical bell shape the value of τ' being largest at pH 7 and decreasing at more acid or more alkaline pH. In acid pH the mepc amplitude (A) decreased relative to its value at pH 7, and in alkaline pH A increased. In Rana ridibunda a narrower and asymmetric bell-shaped dependence of τ' on pH, having a maximum of pH 5.5, was found. The mepc amplitude was again reduced in acid pH but had a peak at pH 5.5. Also, its value at pH 9 was larger than at pH 7. These results were obtained with a number of different buffers and were not found to be sensitive to the nature of the buffer chosen. By performing ACh-noise analysis we found that in Rana pipiens at acid pH (5.5-5.0), the single channel conductance (γ) and the single channel open time (τ) were significantly reduced relative to their value at pH 7. However, in Rana ridibunda at acid pH (5.4) γ was unchanged and τ was markedly increased relative to their values at pH 7. The results can be explained quantitatively by electrostatic interaction between two fixed and titratable ionic groups and a mobile charge in the receptor molecule. The model fits the data for groups having pKs ~4.8 and ~9.8 for Rana pipiens and ~4.6 and ~6.3 for Rana ridibunda. The groups can be tentatively identified as amino acid residues; glutamic or aspartic and lysine or tyrosine for Rana pipiens; glutamic or aspartic and histidine for Rana ridibunda. The difference in the fitted values of the other model parameters for these two species can be attributed to differences in the spatial configuration of the charged groups.