Tomato plants were grown under light intensities of 36 or 90 W m−2 [photosynthetically active radiation (PAR)], and then the light intensity was changed to 36, 90 or 180 W m−2 for 8 h to investigate the effect of temporary changes in light intensity on the carbon budget of photoassimilates from the third leaf using a 14CO2 steady-state feeding method. In the plants that were raised under 90 W m−2, the photosynthetic rate increased when the light intensity was increased to 180 W m−2, whereas no increase occurred in the plants that were raised under 36 W m−2. Although the total amount of carbon fixed during the 8-h light period showed a large difference between plants grown at the two initial light intensities, the proportion of carbon exported during the light period did not differ apparently, irrespective of the change in light intensity. However, the amount of carbon exported during the time course was higher in plants that were raised under 90 W m−2 than those raised under 36 W m−2, irrespective of the change in light intensity. The partitioning pattern of 14C-photoassimilates was not changed by the change in light intensity, irrespective of whether the light intensity was increased or not. However, the amount of 14C-photoassimilates accumulated in each part differed according to the two initial light intensities. The carbon transport from a source leaf was also investigated through a quantitative analysis of carbon balance.