Ricinus communis plants were grown under normal (350 ppm) and elevated (700 ppm) CO2 atmosphere and the growth and carbohydrate status of leaf 2 (first leaf above the pair of primary leaves) was studied. Elevated carbon dioxide stimulated the growth of leaves 1·7-fold. The glucose and fructose concentrations exhibited the same diurnal rhythm under both growth conditions. The sucrose concentrations stayed relatively constant and at 700 ppm were one-third higher than at 350 ppm. The starch content increased steadily during the day and disappeared overnight at 350 ppm CO2, but remained partially in plants at 700 ppm CO2. Consequently at 700 ppm CO2, the leaves accumulated starch continuously over their life time. The rate of starch synthesis was correlated to the activity of ADP-glucose pyrophosphorylase, which was related to the sucrose concentration in the leaf. It is concluded that sucrose controls the expression of ADP-glucose pyrophosphorylase, leading to a shift of carbohydrate partitioning into starch when more sucrose is produced than consumed or exported, a situation which is especially pertinent at elevated CO2. These results show that the previously experimentally observed transscriptional regulation of starch synthesis by sucrose occurs in vivo in the daily life of a leaf.