After cooling the roots and shoot apex of barley, there is a reduction in sucrose export from, and an increased total accumulation of carbohydrate and induction of fructan synthesis in, source leaves. We investigated carbohydrate accumulation in individual leaf cells. Using the microsampling technique and microfluorometric enzymatic assay, the amounts of various carbohydrates were measured in mesophyll, bundle sheath and epidermal cells. Epidermal cells take no part in carbohydrate partitioning and have very low concentrations of sugars even when leaf export is reduced. Even without cooling, however, there are remarkable changes in sucrose concentration (up to 200 mM) in mesophyll and bundle sheath cells during the photoperiod. The activity of acid invertase in samples from individual cells was very low, c. 3 μmol sucrose (g f. wt)−1 h−1. Interrelationships between these cell types and the control of fructan metabolism are discussed.