The magnitudes of the electrical potential and proton gradient in Methanospirillum hungatei GP1 and Methanobacterium thermoautotrophicum were determined. No delta pH (inside alkaline) could be demonstrated in either organism suspended in growth media at normal growth pH values by the distribution of 5,5-dimethyl-2,4-oxazolidinedione (DMO), butyrate, propionate, or methylamine. The internal pH, estimated to be approximately 6.7 under our growth conditions, was not constant, but varied as the external pH was adjusted. However, the internal pH was always more neutral than the external pH (except at pH 6.7 where the two were equal). The distribution of triphenylmethylphosphonium cation, in the presence of tetraphenylboron anion, gave estimates of 119 and 79 mV (interior negative) for the electrical potentials of the thermophile and mesophile, respectively, for cells suspended in a phosphate buffer (pH 7.0). The uptake of 86Rb+, in the presence of valinomycin, gave similar results for M. thermoautotrophicum, ranging from 143 mV (at pH 5.8) to 120 mV (at pH 8.0). Electrical potentials compared to the size of the respective K+ gradients, maintained between the cytoplasm and growth medium. The results are interpreted in terms of proton efflux and monovalent cation antiport activities at the cytoplasmic membrane, with possible proton pumping at the site of internal vesicles.