Root biomass of rice seedlings was increased at lower concentration of exogenous NH+4, but it was decreased at higher concentration of exogenous NH+4. The level of free NH+4 in the roots was accumulated gradually with the increase of NH+4 concentration in the nutrient solution. The content of the soluble proteins was essentially constant at higher NH+4. The activities of glutamine synthetase(GS), NADH-dependent glutamate synthase(NADH-GOGAT), and NADH-dependent glutamate dehydrogenase (NADH-GDH) were risen with exogenous NH+4 concentration at the lower NH+4 concentration range. But the activities of GS and NADH-GOGAT were declined, and the level of NADH-GDH activity was kept constant under higher NH+4 concentration. The GS/GDH ratio suggested that NH+4 was assimilated by GS-GOGAT cycle under lower NH+4 concentration, but NADH-GDH was more important for NH+4 assimilation and detoxifying NH+4 to the tissue cells at the higher NH+4 level. According to the growth and the activity changes of these ammonium-assimilating enzymes of rice seedling roots, 10.0μg/mL NH+4-N in nutrient solution was more suitable to the rice growth.