Sucrose in barley leaf blades can be considered as being compartmented between two types of pool: transport (including mesophyll cytosol and apoplast, and vascular tissue) and vacuolar. The relative sizes of these pools, and their changes over a light/dark cycle, have been estimated using 14C labelling of the intact leaf and washout of 14C from leaf discs. Both types of pool increase during the photoperiod, the proportional distribution between them remaining the same; roughly one-fifth of the sucrose is in the transport pool. The rate-constants describing translocation, and loading and unloading of the vacuole, show diel changes, and fluxes of sucrose across the tonoplast are likely to be critical in controlling fluxes of photosynthetically fixed carbon within and out of barley leaves. The unloading of sucrose from vacuoles is probably the site where control is exercised; this conclusion is supported by a simple simulation model.