Segments of leaves of Vallisneria spiralis L. were allowed to take up CO2 and HCOs-ions and assimilate the inorganic carbon either in the light or in the dark. C4-fixation occurred in the dark producing malate and aspartate as the first compounds, but some residual activity of the Calvin cycle could be detected, too. Both C4 and C3 fixation were stimulated by light, the C4 fixation amounting to 20-25% of total fixation rate under the prevailing conditions. By extending the dark fixation period a label distribution pattern could be obtained which hardly deviated from the pattern found in a light fixation experiment. Label from aspartate was transferred to other amino acids and to organic acids, citrate-isocitrate being the most apparent one. No detectable activity could be demonstrated in oxalic acid, which is the most abundant acid in Vallisneria leaves. In contrast, malate is accumulated and no turn-over could be demonstrated. From preliminary data on label distribution within the malate molecules we deduced that only single-labelled molecules were formed in the dark. The label tended to be distributed evenly among the fourth and first C-atom. However, in the light there was a gradual tendency to uniform labelling, i.e. more C-atoms became labelled. Transfer of labelled atoms between C3 and €4 products must have occurred in both directions. However, no detectable amounts of labelled PEP, which seemed to be a plausible intermediate in the transfer reactions, could be demonstrated. The accumulated malic acid may function as a C02-reservoir that can be drawn on under conditions of reduced CO2 supply from the medium, which is, in turn, the results of the highly reduced diffusion of CO2 in a liquid medium which leads to rapid depletion of the solution contacting the leaves.