The present work was undertaken to investigate the effect of nitrogen top-dressed at the different growth stages in rice plants on its leaf area growth and on the nitrogen levels in the individual leaf blades, and furthermore to establish nitrogen translocation and distribution into the various plant parts. For these purposes, ¹⁵N-labelled ammonium sulfate was used as a tracer in order to measure absorption of nitrogen fertilizer, and to follow the translocation of absorbed ¹⁵N-labelled nitrogen into grains from the other plant parts. The results obtained are as follows. 1. A marked increase was found in leaf area growth of the rice plant by top-dressing of nitrogen during the growth stages of tillering to branch differentiation, but it was not found to occur after the boot stage. During the ripening period a decrease of the leaf area growth was significantly suppressed by top-dressing. The contents of total nitrogen, soluble and Fraction-1 proteins in the leaf blades greatly increased by top-dressing after the boot stage and these values were much higher than the corresponding values at the tillering and branch differentiation stages. 2. The leaf area growth of individual leaf blades was effectively promoted when nitrogen was top-dressed sometime before the emergence of the leaf in question. For example, the growth of 12th and 13th leaf area was more accelerated by top-dressing at the 11th leaf elongating stage and the growth of 13th and 14th leaf area by top-dressing at the 12th leaf elongating stage. However, after the boot stage nitrogen fertilization was ineffective on the leaf area growth. 3. Of the two rice varieties used, Kamenoo showed a relatively higher growth of leaf area as compared with Hatsukaori. By top-dressing of nitrogen, the increased leaf area growth was more obvious in the former variety than in the latter one. The extent of leaf area growth was marked when nitrogen fertilizer was top-dressed at the tillering or branch differentiation stages. On the contrary, Hatsukaori showed higher levels of total nitrogen, and soluble and Fraction-1 proteins in the leaf blades rather than increasing the leaf area growth by top-dressing of nitrogen. 4. The translocation of top-dressed nitrogen into individual leaf blades differed considerably with the growth stages at which labelled nitrogen was supplied. ¹⁵N-labelled nitrogen supplied at the tillering stage (11th leaf stage) tended to accumulate into the just elongating 12th leaf blades whereas ¹⁵N supplied at the stage of branch differentiation stage (12th leaf stage) was predominatingly translocated and deposited in the upper 13th and 14th leaf blades. However, after the boot stage little or no difference was found in distribution of ¹⁵N-labelled nitrogen among the individual leaf blades, and ¹⁵N-labelled nitrogen tended to accumulate mainly into the developing ears rather than the other plant parts (leaf blades and leaf sheath+culms). 5. The percentages of ¹⁵N distributed to sink were 52, 58, 70 and 72 for nitrogen top-dressed at the tillering, branch differentiation, boot and heading stages, respectively. These data agreed with the results that the grain-N content together with glutelin (one of the reserve proteins) increased markedly by top-dressing of nitrogen at the boot and heading stages.