The influences of different NH₄⁺-N Ievpls on the nitrogenase activity in rhizosphere of rice under the ecological condition of Hangzhou were observed. The results obtained are as follows:(1) NH₄⁺-N significantly inhibites nitrogen-fixing activity in rhizosphere of rice within a certain time. There was a highly (or medium) negative currelation between the concentration of NH₄⁺-N in soil and the nitrogen-fixing activity in rhizosph ere of rice. During different grnwing stages, values of correlation coefficient ranged from -0.4428 to -0.9945. (2) The initial concentration of NH₄⁺-N in soil which began to inhibite the nitrogen-fixing activity in cores of paddy soils was 20 ppm. (3) The time duration of the inhibiting effect of NH₄⁺-N varied with the rate of NH₄⁺-N applied. The inhibiting effect lasted about 20 days in low nitrogen plots, and about 25-30 days in medium and high nitrogen plots. Since then, the application of NH₄⁺-N showed favorable effect on the nitrugen-fixing activity in rhizosphere of rice.