The aim of this work was to examine whether carbohydrates are involved in signalling N deficiency through source:sink imbalance. Photosynthetic metabolism in tobacco was studied over 8 d during the withdrawal of N from previously N-sufficient plants in which the source:sink ratio was manipulated by shading leaves on some of the plants. In N-sufficient plants over this time-scale, there was a small decline in photosynthetic rate, Rubisco protein and amino acid content, with a larger decrease in carbohydrate content. Withdrawal of N from the growing medium induced a large decrease in the rate of photosynthesis (35% reduction after 8 d under the growing conditions, with a reduction also apparent at high and low measuring CO2), which was caused by a large decrease in the amount of Rubisco protein (62% after 8 d) and Rubisco activity. Higher amounts; of hexoses preceded the loss of photosynthetic activity and sucrose and starch accumulation. Reduction of the sourcersink ratio by shading prevented the loss of photosynthetic activity and the increase in hexoses and other carbohydrates. These data indicate that the reduction of photosynthesis that accompanies N deficiency in intact plants has the characteristics of sugar repression of photosynthesis observed in model systems, but that the accumulation of hexose prior to the decline in photosynthesis is small. The possibility that sugar repression of photosynthesis under physiological conditions depends more crucially on the C:N status of leaves than the carbohydrate status alone is discussed.