The Mg2+, K+ and Cl- contents of chloroplast pellets collected after centrifugation in light and dark conditions have been determined in a specific reaction medium at pH 7.8 and 6.6. It was found that the light-induced proton uptake by chloroplasts was electrically balanced mainly by Mg2+ efflux and to a lesser extent by K+ efflux. Unlike results in other studies, Cl- influx was rather small. A model for the ionic relations of envelope-free chloroplasts is considered. Light-induced changes in the ionic contents calculated from the model are in agreement with our observations. An attempt is made to apply similar considerations to another set of reported data.