Exogenous Mg(2+) (2 millimolar) altered the stromal pH of intact spinach chloroplasts. Without added KCl in the medium, Mg(2+) decreased the stromal pH in the light by approximately 0.3 pH unit. External KCl (25 millimolar) largely prevented the acidification caused by Mg(2+). Effects on the stromal pH were not caused by changes in H(+) pumping across the thylakoid membrane because Mg(2+) had no effect on the light-induced quenching of atebrin fluorescence by intact chloroplasts. However, Mg(2+) affected H(+) fluxes across the envelope. Addition of Mg(2+) to intact chloroplasts in the dark caused a significant acidification of the medium that was dependent on the presence of K(+).External K(+) or Na(+) also prevented the inhibition of CO(2)-dependent O(2) evolution by Mg(2+), whereas choline chloride was less effective. The combination of Mg(2+) and K(+) stimulated O(2) evolution at suboptimal pH, inhibited O(2) evolution at optimal and superoptimal pH, and prevented the inhibition of photosynthesis caused by acetate. In the absence of added K(+), Mg(2+) was most inhibitory to O(2) evolution at suboptimal pH.The results suggested that Mg(2+) activated a reversible (Na(+))K(+)/H(+) exchange across the chloroplast envelope. It is postulated that changes in the stromal pH may explain the inhibition of photosynthesis caused by the presence of exogenous Mg(2+).