Ion movements associated with the pH rise that is observed upon illumination of thylakoid suspensions at low pH have been studied by a multiparameter technique. Light-dependent, dark-reversible fluxes of H+, Cl-, Na+, K+ and divalent cations were monitored, together with simultaneous changes in the optical density of the suspension. Extensive uptake of Cl- and efflux of Mg2+ accompany the apparent inward movement of H+ in the light. Only minor efflux of K+ is seen and Na+ appears immobile. The Cl- and Mg2+ fluxes together compensate for most of the charge transferred as H+, contributing respectively about 49% and 43% on an equivalent basis. The ratio of Cl- influx to Mg2+ efflux is variable, but usually >1.0. The Mg2+ flux can be supplanted by (1) K+ flux, if the K+/Mg2+ activity ratio in the suspension is high, and (2) Ca2+ flux, if the thylakoids are equilibrated with suspending media containing Ca2+. The affinity of the divalentcation-binding sites, or carrier mechanism, is greater for Ca2+ than for Mg2+. Schemes can be drawn up to account for the observed ion movements on the basis of either a chemical or a chemiosmotic mechanism for energy transduction in chloroplasts. In intact chloroplasts, light-dependent control of Mg2+ distribution between thylakoid and stroma could serve to regulate enzyme activities in the carbon fixation pathway, and hence photosynthesis.