A technique for the estimation of light-induced membrane potential in chromatophores is described. It is based on measurement of light-induced enhancement in fluorescence of 8-anilinonaphthalene sulfonic acid, which is calibrated by known K + diffusion potentials. The electrochemical proton gradient (Δ μ H+ − ) formed during lightinduced electron transport in Rhodospirillum rubrum chromatophores amounts to 250 mV, which is almost equally distributed between the membrane potential and the pH gradient as measured by changes in the fluorescence of anilinonaphthalene sulfonate and 9-amino acridine. Addition of the permeant anion, NaSCN, or of NH 4Cl reduces the overall Δ μ H+ − by less than 20% but changes its distribution between the pH gradient and the membrane potential so that with NaSCN it is composed mainly of the first and with NH 4Cl mainly of the second. Initiation of phosphorylation causes a drop of about 50 mV in the measured Δ μ H+ − . In the absence of salts, the drop is observed in both components, although two-thirds of it are reflected in the membrane potential. In the presence of NaSCN or NH 4Cl the 50-mV drop is exclusively recorded in the pH gradient or in the membrane potential, respectively. The steady-state phosphate potential maintained during electron transport was found to change in parallel to the Δ μ H+ − , but exceeded it by 60 to 80 mV when based on a stoichiometry of two protons translocated per ATP synthesized.