The single rooted leaf of soybean (Glycine max L. Merr.) was used to study source-sink relationships in photosynthesis. When the leaves were kept under a regime of 10 h light (410–480 μmol photons m⁻², 400–700 nm)−14 h dark, they did not expand, the increase in leaf dry weight almost stopped, and photosynthetic activity remained at a high and constant level for 8 d while the dry weight of the roots increased at a constant rate throughout the period. Thus, under this condition the leaf and the root system served as the only source and sink organs, respectively. When leaves grown for 7 d under this condition were placed under continuous light to alter the source/sink balance in photosynthate, the root dry weight increased at a constant rate equal to that found under the 10 h light−14 h dark condition. The leaf dry weight markedly increased and by day 5 of continuous light had increased 1.6-fold, mainly as a consequence of accumulation of starch and sucrose, which were not translocated for root growth. The continuous light caused an abrupt decrease in the photosynthetic activity (40% of initial value by day 5). However, the activity recovered almost completely after a 32-h transfer to darkness. Significant negative correlations existed between photosynthetic activity and the sucrose and starch contents in the rooted leaves placed under continuous light. When the plants were treated with various light conditions, there was no significant difference (p<0.01) among the regression line slopes for photosynthetic activity on the sucrose content, but there was some deviation among those for the photosynthetic activity on the starch content. This result suggests that sucrose accumulated in the leaf has a more direct influence on photosynthetic activity when the source/sink balance was altered.