Ananaerobic incubation and field experiment of fertilize-rN application of different rates were conducted in the Taihu region to determine effects of fertilize-rN on soil N supply, rice N uptake and groundwater.The results show that soil N availability could be considerably enhanced after years of N-fert ilization.N content in rice and N uptake by rice increased with the N application rate, but the rate of N transferred to grain decreased.The optimum fert ilize-rN application rate ranged from N225 to 270 kg hm^-2 for rice in this region.The concentrations of both NH₄⁺-N and NO₃^--N in the surface water of paddy field rose temporarily as a result of fertilize-rN application, but they were not significantly correlated.NO₃^--N was the main form of nitrogen in the leachate from paddy field.Its concentration rose quickly arriving at a peak around 1.62~2.75 mg L^-1 within the first 7 days after the field was flooded for transplanting, and then dropped to below 0.5 mg L^-1 after 10 days or so.NH₄⁺-N concentration in the leachate seemed increasing with the N application rate and peaked at the end of the rice tillering stage, but usually remained lower than 0.2 mg L^-1 in the rest of the growing season.