Soybean plants were grown in N free nutrient solution. At the pod setting (P.S.) or initial pod filling (J.P. F.) stage, plants were fed with 16 NOa-(Nt-plants) or 1 •NH4+ (Am-plants) in order to investigate the utility of nitrogen top dressing. P.S. and I.P.F. as well Nt-and Am-plants were compared.The absorption and incorporation of combined nitrogen in the ethanol soluble and insoluble fractions of different plant parts were higher at P.S., particularly with the nitrate form. Among the different plant parts, pods represented the biggest sink of combined nitrogen at both P.S. and I.P.F.Both Nt-and Am-plants showed almost the same pattern in the percentage distribution of combined nitrogen in the ethanol soluble and insoluble fractions of pods. The distribution percent of combined nitrogen in the upper leaves was higher than that in the lower leaves, particularly in the ethanol soluble fraction in Nt-plants and the insoluble fraction in Amplants. In stems, roots and nodules, the distribution percent of combined nitrogen was higher at I.P.F., particularly in the ethanol soluble fraction.At I.P.F., combined nitrogen showed a higher incorporation into pod protein, while the opposite was true for other plant parts particularly sterns. Am-plants revealed a higher incorporation of combined nitrogen into root protein.Among the different plant parts, the roots exhibited the highest percentage of combined nitrogen in the total nitrogen content, particularly in Nt-plants, while the nodules were much less dependent on combined nitrogen than the other plant parts.Based on the above results, it is suggested that P.S. may represent a suitable period for the combined nitrogen top dressing of soybean plants during their pod development.