The effect of a pH gradient (Delta pH) and a membrane potential difference (Delta psi) on the rate of proton translocation by bacteriorhodopsin was examined. Contrary to the "proton well" hypothesis, variation of Delta psi, exerted a considerably larger effect on the rate of proton translocation than the energetically equivalent magnitude of Delta pH (maximum Delta pH examined was about 2, while the absolute pH value was 5-9.) These apparently puzzling features are, however, consistent with the structural data, particularly in view of an asymmetric environment provided by the key amino acid residues with different pK(a) values. The relatively small effect of Delta pH is explained in terms of the proton uptake residue, Asp96, and the proton-ejecting residue, Asp85, whose pK(a) values are known to be about 10 and 3 in the ground state, respectively. On the other hand, proton transfer from Asp96 to the Schiff base during the decay of the M intermediate can account for the large effect of Delta psi on the rate of proton translocation. With these experimental data and explanations in mind, we further propose a simplified stochastic model for proton pumping where an asymmetric environment, which in turn provides an asymmetric potential field for protons, plays an essential role for vectorial proton translocation. A simple numerical simulation could qualitatively reproduce the experimental data. These results suggest that some common principle may exist in the mechanisms of ion pumps and molecular motors, and it may be applied in development of an artificial ion pump molecule.