Soluble carbohydrate accumulated faster in second leaf blades of barley when plants were grown in 700 μmol mol−1 CO2 rather than 350 μmol mol−1 CO2. Infection of the second leaf blade by powdery mildew had no effect on the concentration of soluble carbohydrate until 6 d after inoculation when it was lower than in controls. The accumulation of soluble carbohydrate in the second leaf of uninfected plants grown in 700 μmol mol−1 CO2 was due largely to earlier and faster accumulation of fructan. TLC showed that the series of fructan was not different in plants grown in 700 μmol μmol−1 CO2 relative to plants grown in 350 μmol mol−1 CO2, neither did infection by powdery mildew affect the series of fructan present in the second leaf blade. The rate constant for phloem loading obtained by compartmental analysis of 14C efflux from the leaf blade was not reduced in plants grown in 700 μmol mol−1 CO2, indicating that carbohydrate accumulation was not caused by reduced ability of the leaf to export carbon.