The formation of labelled phosphate esters in cytoplasm and chloroplasts of leaf cells of Elodea densa has been followed in feeding experiments with radioactive bicarbonate and phosphate in the light and in the dark. From the kinetic analysis of the distribution of label between chloroplasts and cytoplasm it is concluded that the incorporation of 14C into sugar phosphates and phosphoglyceric acid proceeds in the light exclusively, or almost exclusively, in the chloroplasts. On the other hand, in the light and in the dark incorporation of 32P into sugar phosphates, ATP and ADP occurs predominantly in the cytoplasm of the leaf cell. This can only be explained in terms of low permeability of the chloroplast membrane to inorganic phosphate. Due to this a specific activity of inorganic phosphate, which is higher in the cytoplasm than in the chloroplasts, is maintained during the feeding experiments. This results in higher incorporation of 32P into organic compounds in the cytoplasm, although phosphate turnover is slower there than in the chloroplasts. In consequence, 32ΡΟ43Θ appears unsuited for many types of in vivo experiments on the phosphate metabolism of photosynthesis. Contrary to other phosphate esters, phosphoglyceric acid becomes labelled with 32P, as with 14C, in the light predominantly in the chloroplasts. This indicates a light inhibition of phosphoglyceric acid formation in the cytoplasm in addition to the inhibition of the citric acid cycle. ATP levels of the cell increase markedly upon illumination and decrease rapidly after darkening. That the response of the ATP/ADP system to light and dark is much faster than that of inorganic phosphate strongly suggests a control of respiration in photosynthesizing cells by the decreased steady state ratio of ADP to ATP rather than by lack of inorganic phosphate.