Nuse efficiency of rice is key to its overall yield potential,and efforts to improve the key potential must be guided by a thorough understanding of the process that governs N use efficiency.Field experiment was conducted to assess differences in grain yield,Naccumulation,and N physiological use efficiency between 177 genotypes of rice japonica(71 semilate varieties(130~150 d) and 106 late varieties(151~172 d) under two levels of N supply(0 and 180 kg hm^-2) in 2003.Grain yield and N accumulation increased with the N rate,while physiological N use efficiency decreased.Significant differences in grain yield,N uptake and N physiological use efficiency were observed between genotypes within the same variety group under the same N application rate.Based on grain yield under two N rates,genotypes within each group were divided into four types,such as efficient-efficient,inefficient-efficient,inefficient-inefficient,and efficient-inefficient.High N use efficiency(NUE) cultivars produced high yields when N was applied,and low NUE cultivars low yields.Thus genotypes which have promising traits for improved N uptake and N physiological use efficiency are identified.Those with higher N physiological use efficiency were lower in N concentration in straw at maturity.Further study is needed to elucidate mechanisms involved in high N uptake and N physiological use efficiency.