The fate of top-dressed nitrogen (ammonium sulphate) enriched with ¹5N was investigated in the paddy field. The amounts of nitrogen applied were 30 kg per hectare in 1969 and 25 kg per hectare in 1970, respectively. The earlier was the top-dressing at the growth stage of rice plant applied, the longer stayed the ammonia derived from the top-dressed fertilizer in the plowed layer. This fact is approximately consistent with the duration of absorption of top-dressed nitrogen by rice plant. After harvest only 3 to 10 per cent of top-dressed nitrogen was found in the plowed layer. Since the drainage of the paddy soil was very poor, it was assumed that almost all of the unrecovered top-dressed nitrogen was lost by denitrification. The recoveries of top-dressed nitrogen were higher for the plots applied at the young panicle formation stage than for other ones. The earlier was the top-dressing applied, the longer the duration of top-dressed nitrogen uptake by the plant. And the later was the top-dressing applied, the higher the absorption rate of nitrogen by the plant. The amounts of soil nitrogen abosrbed by the plant were greater in all the plots with top-dressing nitrogen than in those without top-dressing. Therefore, the gross recovery of nitrogen in the plant was greater than the recovery calculated from the tracer.