The H+-ATPase from chloroplasts was brought into the active, reduced state. Then, an electrochemica| potential difference of protons across the thylakoid membranes was F_,cnerated b~ ,in acid-base transition, ~lpH, combined with a K*/valinomycin diffusion potential. ~. The initial rate of ATP synthesis was measured with a rapid-mixing quenched-flow apparatus in the time-range between 20-150 ms. The rate of ATP symhesis depends in a sigmoidal way on ,dpH. Increasing diffusion potentials shifts the ApH-dcpendcncies to ~ower ~pH values. Analysis of the data indicate that the rate of ATP synthesss depends on the electrochemical potential diffcrenoe of protons irrespective of the relative contribution of ApH and Ch!oroplast; H~-ATPase; Enzyme-kinetics