This study aims to characterize the translocation of photosynthates within and from developing tall fescue (Festuca arundinacea) leaves at the time of transition from sink to source. The developing leaf contains a source, the exposed tip, and a sink, the growing basal portion. When the exposed tip of the developing blade is labelled with 14CO2, it exports photosynthates exclusively to sinks within the developing blade until the blade reaches 80% of its final length, when photosynthates begin to be exported from the blade and pass through the collar to reach the growing sheath and the next expanding leaf. Concurrently, the previous mature leaves reduce their level of photosynthate export to the developing blade; export stops as soon as the sheath of the developing leaf elongates beyond 10 mm. Export from the mature leaves to the growing sheath and to the next expanding leaf blade increases rapidly. Thus, in a developing tall fescue leaf blade photosynthate importation and exportation are exclusive events: the expanding blade imports photosynthate from mature leaves until it reaches 80% of its final length, then exportation begins and importation ceases.