The effect of light on anion uptake by detached leaves of Limnophila gratioloides was investigated using different external concentrations of chloride and sulfate. The results show that the effect of light may be dependent not only on the plant species 1-4,8,9 and more or less imponderable conditions like the nutritional and developmental state of the plant but also on the external ion concentration and the duration of the uptake period. Chloride uptake by Limnophila leaves from solutions of 0.1 - 1.0 mM/I was not affected by light (4000 Lux), uptake from 1.0 - 10.0 mM/l, by contrast, was stimulated by light in an initial uptake period (4 h), i. e. the k m of this system was decreased by light leaving v max unaltered. Sulfate uptake was inhibited by light if the external sulfate concentration was within the range of 0.01 -7.0 mM/l. Applying external sulfate concentrations from 7.0 - 50.0 mM/l results in stimulation of uptake by light. A workable approximation to these observations may still be that the primary metabolic anion uptake, particularly chloride uptake, into leaf cells is mediated by transport systems which are remote from the mitochondria and chloroplasts and which are supported by a cofactor produced either in oxydative phosphorylation, in cyclic plus noncyclic photophosphorylation or in cyclic photophosphorylation alone 1-4, 9 . Since kinetic data are equal in light and in dark, direct coupling of chloride uptake to electron flow in mitochondria or chloroplasts is unlikely. The hypothesis is developed that light may exert a dual influence on ion uptake: 1. Metabolic uptake may be influenced by a change in the availability of ATP to transport systems. 2. Passive fluxes and counterfluxes of ions across cell membranes may be affected by production or translocation of intracellular ions. Since chloroplasts represent important intracellular compartments, light-induced translocations of ions 22 (including Ca 2⨁ 23 !) across the chloroplast membranes may exert a marked influence on the overall ion uptake by stimulation of the passive influx of ions across the plasmalemma 15 . In line with this views is the observation that in Limnophila leaves, which contain approximately equal ATP levels in light and in dark, there is no stimulation of chloride uptake by light at low external concentrations of KCl while at high external concentrations, at which passive fluxes of ions across the plasmalemma become effective, both ion uptake and ion loss is stimulated by light. The susceptibility to the external ion concentration of the influence of light suggests that effects of light on ion transport ought to be studied at various external ion concentrations. The applicability of imidazole as uncoupler of photophosphorylation in vivo is discussed.