Light-dependent formation of ribulose-1,5 diphosphate is completely inhibited by low concentrations of 3-(3,4-dichlorophenyl)-1,1-dimethylurea which do not severely affect cyclic photophosphorylation. Also in Scenedesmus mutant number 11, capable of cyclic photophosphorylation, cellular ribulose-1,5 diphosphate-levels do not increase upon illumination. When mutant cells are H(2) adapted, however, a light-dependent formation of ribulose-1,5 diphosphate is observed in the presence of H(2). From these results it has been concluded that at least part of the Calvin cycle does not operate in the dark, since a reductant is lacking which is generated in the light.