Petioles of Sinapis cotyledons cultured in 6×10−2 M sucrose in the light increased 28-fold in total carbohydrate content over a 7-d period compared with an 8-fold increase in petioles of cotyledons cultured on water. The starch and reducing sugar fractions were the major components of this accumulation. Labelled sucrose applied to the petiole base moved quickly up the petiole and into the main veins of the lamina. Some basipetal redistribution occurred subsequently and after 24 h radioactivity accumulated strongly at the petiole base. Culture in sucrose reduced basal accumulation and increased acropetal movement of the label. Fixation of 14CO2 by petioles remained constant when cotyledons were cultured in water, whereas in sucrose, fixation fell by 50 per cent during the first 2 d. The pattern of inhibition of fixation matched the pattern of sucrose distribution in the petiole. Petiolar chlorophyll content remained constant in controls but fell rapidly after 4 d culture in sucrose. The results are discussed in relation to the role of petiolar carbohydrate accumulation in the regulation of CO2 fixation, primordium development, and senescence in this system.