Nitrate is becoming more and more important for N nutrition of rice plants since rice is now cultivated in intermittent irrigation or even in aerobic soil condition. Moreover, the rhizosphere of rice roots is actually in partial oxidized status due to the released oxygen by rice roots. Solution culture experiments were carried out to study the effect of NO₃^- on the NH₄⁺ uptake kinetics of rice roots and the glutamine synthetase activity (GSA) in leaves of four typical rice genotypes (Conventional Indica, Conventional Japonica, Hybrid Indica, Hybrid Japonica) at the seedling stage (28 day old). The results obtained were as follows. Partial replacement of NH₄⁺ by NO₃^- could improve the growth and N uptake of rice. The rate of NH₄⁺ uptake by dif ferent genotypes was in following order:Hybrid Indica > Conventional Indica> Hybrid Japonica > Conventional Japonica. The presence of NO₃^- could increase the NH₄⁺ uptake of rice. This increase was mainly attributed to a significantly increased V_max (31.5% on average of the four genotypes) in the treatment with NO₃^- while the K_m values for NH₄⁺ (the increased percentage being 4.26% on average of the four genotypes) were not significantly affected by the addition of NO₃^-. NO₃^- could also increase the GSA in the leaves of rice, and thus, the assimilation of NH₄⁺ were improved.