The apportioning of newly fixed carbon in leaf discs of sugar beet (Beta vulgaris L.) is dependent on the incubation medium. Concentration of orthophosphate (Pi)* influences the distribution of carbon between chloroplast starch and the cytoplasm, and the proportion of 14C incorporated into sucrose. In the presence of exogenous sucrose and glucose, starch synthesis is enhanced and gross changes in cytoplasmic metabolite pools occur. These observations are discussed in relation to sink/source interactions of whole plants. In the presence of mannose, which sequesters cytoplasmic Pi in leaves of sugar beet as mannose-6-phosphate, sucrose synthesis is severely inhibited and maltose accounts for up to 30 % of the fixed carbon. This is related to earlier reports of unusual accumulation of maltose.