[en] The translocation and distribution of nitrogen top-dressed at early ripening stage of rice plants (Oryza sativa L. cv Chukyoasahi) were studied, using N-15 as a tracer. The plants cultured with Kasugai nutrient solution were top-dressed with N-15 labeled ammonium sulfate 7 days after heading. The heading was designated as the flowering day of the uppermost spikelets on panicles. The incorporation of N-15 was detected in culms, leaf sheaths and panicles 3 hr after the top-dressing, but in leaf blades, it was detected 3 hr later. The amount of N-15 in these organs increased markedly up to 48 hr after the top-dressing, then began to decrease, but the amount of N-15 in panicles continued to increase throughout the experiment period. It is likely that the amount of N-15 in panicles was not transported from leaf blades, but directly transported from roots. Much of N-15 was distributed to early developing grains on primary branches, and this distribution pattern was coincident with the differentiation order to spikelets on the inflorescence. In the grains on top and middle primary branches, the incorporation of N-15 was detected 3 hr after the top-dressing, while in the grains on bottom primary branches, it was detected 6 hr after the top-dressing. The concentration of N-15 in grains was about 10 to 15% lower than that in whole panicles during the period of active absorption and translocation of top-dressed nitrogen. (Kako, I.)