Results of the studies, including relevant methodologies, on nitrogen fixation and metabolism in soybean plants which have been undertaken in Japan in the past 10 years are reviewed in this paper. Three methods, i.e. ¹⁵N₂ gas feeding, ¹⁵N dilution and ¹⁵N natural abundance, were employed for estimating an amount of fixed nitrogen. In the two isogenic line of soybean: non nodulating (T201) and nodulating (T202) lines, the ratio of symbiotically fixed N in the total absorbed N were estimated to be 76% by the ¹⁵N dilution method, while it was 52% by the ¹⁵N natural abundance method. Contribution of the following three N sources; fertilizer, N₂ fixation and soil-N, was estimated with a ¹⁵N tracer technique in combination with ¹⁵N natural abundance and with low level ¹⁵N application. Nodulating soybean plants obtained 13±10% of their N from fertilizer, 66±8% from N₂ fixation and 21±10% from soil N in Andosol. In the experiment in which nodules attached to the intact roots was exposed to ¹⁵N₂, it was observed that the fixed N was rapidly translocated from bacteroids to plant cell cytosol in the form of ammonia, and assimilated by the GS/GOGAT system in the latter.