Three varieties of early rice, namely Dwarf-Nanta, Lo-tsai and 503 were studied for their endurance at various rate of application of ammonium sulphate. Their root characteristics and leaf composition were analysed. Positive correlations have been found between the endurance of rice plants to heavy nitrogen fertilizer and the absorption surface and ration exchangeable capacity of their roots. The order of stability of the three varieties of rice at heavy nitrogen application is Dwarf-Nanta>Lo-tsai>503. Dwarf-Nanta showed a slight increase of silica in leaves with an increased rate of nitrogen supply. On the contrary, the content of silica in leaves decreased with increasing supply of nitrogen in Lo-tsai and 503. Under high nitrogen application, the contents of protein-N and chlorophyll in the leaves of Dwarf-Nanta, presumably due to its strong nitrogen assimilation power, also appeared higher than the other two varieties.