Nitrogen is one of the most important nutrients affecting production of cereal crops. Evidence available shows that although nitrate (NO₃^-) enhances growth of rice, the crop prefers ammonium (NH₄⁺ ) to NO₃^-. A hydroponic experiment was carried out to study effects of two NH₄⁺/NO₃^- ratio (100/0 and 75/25) on response of rice different in responsivility to nitrate in root growth at the seedling stage. Four rice genotypes were tested, i.e. Nanguang, Elio (low in NO₃^- responsivity), Shanghai-97 (low in nitrate responsivity) and Liaojing (low in nitrate responsivity). Compared to supply of sole NH₄⁺, enhanced NO₃^- supply increased root biomass and nitrogen accumulation of Nanguang (high in NO₃^- responsivity) by 50% and 79%,respectively, and the total length and number of roots, adventitious roots and lateral roots of the plant increased significantly, however, the mean length of adventitious and lateral roots did not vary much. No such significant effects were observed on other three genotypes of rice. It is clearly indicated that enhanced NO₃^- nutrient stimulates emergence of adventitious and lateral roots only, but does not promote their elongation. From the findings it is inferred that responsivity of rice to NO₃^- in root development differs with the genotype and is one of the factors that affect nitrogen use efficiency by rice.