The influence of DCMU on the photosynthesis and the uptake of chloride and rubidium ions by Vallisneria leaves was studied. The photosynthesis was more reduced by DCMU than was the ion uptake. The rubidium uptake is somewhat more sensitive to DCMU than the chloride uptake. The provisional conclusion is that both the rubidium and chloride uptake can be supported by cyclic photophosphorylation.