Floating rice plants adapt to submergence by rapid internodal elongation. In this study, we examined the changes in the distribution of photoassimilates using ¹³C and the activities of enzymes involved in sucrose metabolism of floating rice plants under submergence. Total (per plant) and specific (per soluble protein) activities of acid invertase (AI) of the rapidly elongating 9th internode increased remarkably 1 day after submergence. These increases corresponded to the accumulation of hexose. It is likely that the increased AI activities of the 9th internode in the submerged plant promoted the hydrolysis of sucroce and maintained the turgor pressure in the elongating cells. Total sucrose synthase (SS) activity of the 9th internode in the submerged plant increased markedly after 3 days and corresponded to the accumulation of dry matter. Comparing each value in the submerged plants with the control, the distribution of photoassimilates to the uppermost 9th internode increased considerably, while there was a decline in the 8th internode. These changes were in agreement with the changes in activities of AI and /or SS. Sucrose/hexose ratio of the 9th internode also declined under submergence. These results suggest that the changes in the activities of the enzymes involved in sucrose metabolism may be related to the changes in the distribution of photoassimilates to the sink organs of submerged floating rice plants via the increase in the magnitude of the sucrose concentration gradients which exist between source and sink.