The light-dependent chloride uptake in leaves of Elodea densa and the distribution of chloride ions within the cells were investigated. The active uptake of Cl(-) into the chloroplasts and into the remainder of the cell proved to be enhanced by light to a similar degree. The percentage of (36)Cl(-) (with respect to the total (36)Cl(-)) found in the chloroplasts was low after short times of uptake but increased to about 24% within 5 minutes. From this it was concluded that also in the light the cytoplasm is the primary site of Cl(-) uptake. Thus the latter cannot be driven by electron-transfer reactions within the chloroplasts.Comparative studies on the action of inhibitors and far-red light on chloride uptake and oxygen evolution in Elodea leaves led to the following results: 1. Concentrations of DCMU that almost completely inhibit the oxygen evolution do not affect the Cl(-) uptake in the light in N2 and in air in the absence of CO2. 2. In the presence of 0.5% CO2 in N2 low concentrations of DCMU partly inhibit the light-dependent chloride uptake. 3. The uncoupling agents of photosynthetic phosphorylation, atebrin (quinacrine) and low concentrations of CCCP, inhibit, the Cl(-) uptake while leaving the oxygen evolution unaffected or even enhancing it. 4. Low intensities of far-red light (λmax=717 nm) can support a small light-driven chloride uptake which cannot be inhibited by CO2. On the other hand, at low intensities of 681 nm light CO2 partly inhibits the Cl(-) uptake, presumably by competing for ATP. From these results it is concluded that photophosphorylation provides the energy for the active chloride uptake in the light. Cyclic photophosphorylation seems to be the only source of energy in the absence of CO2 and in far-red light. In the presence of CO2 also the non-cyclic photophosphorylation seems to furnish part of the available energy for chloride uptake. Evidence is given that the uptake measured in these experiments of rather short duration consists mainly of uptake into the cytoplasm and the chloroplasts.It is suggested that chloride uptake might be a useful measure for the contribution of different types of photophosphorylation to the ATP pool in the cell.