Short pulse-chase labeling experiments indicated that there was a considerable difference between soybean and rice in the distribution pattern of 14C to major metabolites during a 300 s chase period. The distribution of 14C to starch at the end of the chase period was largest in soybean but very small in rice. In rice, the distribution of 14C to sucrose was the largest. Starch formation during the initial stages of photosynthesis implied the existence of a different mechanism of starch synthesis and utilization between soybean and rice. The relative intensity of the flow into starch increased by treatment with 2% O2, suggesting that photorespiratory pathway may involve in the regulation of higher starch accumulation in soybean