Soybean (Glycine max L.) is very important crop for food and animal feed.Soybean plants form root nodules and fix N2 by symbiotic association with soil bacteria,rhizobia.We investigated the fate of nitrogen fixed in soybean nodules by tracer experiment with 15N2 gas.The results indicated that the major part of fixed N in bacteroids (a symbiotic state of rhizobia) is excreted rapidly to cytosol of infected cells in the form of ammonia,then the ammonia is assimilated into amino acids via glutamine synthetase/glutamate synthase (GS/GOGAT) pathway.Then the fixed nitrogen is assimilated into ureides,allantoin and allantoic acid,and transported to the shoots via xylem.On the other hand,nitrate absorbed from roots is mainly assimilated into asparagine.