Nitrogen stress compromises realization of yield potential in cereal crops more than any other single factor.For rice is the world s most important crop species, nitrogen adsorpt ion and utilization efficiency by rice is an important issue in agricultural production.Due to inhibited nitrification in the bulk soil of lowland rice field, researches on nitrogen nutrition of rice focus much more on ammonium (NH₄⁺) than on nitrate (NO₃^-).Much evidence has shown that rice can take up not only NH₄⁺-N but also NO₃^--N.The capacity varies with its genotype.The responses of rice to different forms of nitrogen and their physiological effects are poorly understood.The kinetics of NH₄⁺ and NO₃^- uptake by rice plants of different genetypes were studied in nutrient solutions containing either one or both N forms.The results showed that rice plants of both genotypes grew the best in the nutrient solution of mixed N forms though Indica rice had higher N uptake capacity than Japonica rice.Using solution containing a single form of N, the K_m of NO₃^- uptake was higher than that of NH₄⁺ uptake, indicating that the affinity for NH₄⁺ was greater than that for NO₃^-.The V_max of NH₄⁺ uptake was higher than that of NO₃^- in japonica rice while remained the same in Indica rice.When both NH₄⁺ and NO₃^- were present, the NO₃^- uptake was largely inhibited by NH₄⁺, the V_max of NO₃^- uptake in japonica and Iindica rice decreased by half and two-thirds, respectively.The NH₄⁺ uptake was slightly inhibited by NO₃^- in Indica rice while maintained in Japonica rice.