The long- and short-term movement of carbon in a field crop of winter wheat was investigated with the radioactive tracer carbon-14. The flag leaves of individual plants assimilated a pulse of 14CO2 and the plant parts were assayed subsequently. Groups of plants were pulse-labelled four times during the main period of growth—twice before and twice after anthesis. Plants were harvested and assayed twice weekly after labelling and the time-course of the changes in the amount of 14C recovered from the leaves, stems and ears was observed for each group of plants. Concurrently with these long-term studies, other wheat plants were pulse-labelled, then harvested and assayed 24 h later. The partitioning of 14C between leaves, stem, ear and in some cases roots, was measured over the period from the start of stem elongation to the end of grain filling. Two distinct types of relocation of carbon were observed. Carbon assimilated early in the growth of the plant and used in the growth of new leaves was seen to be partly relocated to the ear. Carbon assimilated 8 d after anthesis was partly stored in the stem, and 15 d later relocated to the ear. This relocation corresponded to a decrease in stem dry mass seen in growth analysis. Little other change in the 14C content of the plants occurred, suggesting that most respiration used current rather than stored assimilate.