The effect of N fertilizers on nodulation, nitrogenase, nitrate reductase activities and growth of two cultivars of soybean, Clark and Crauford was evaluated in a field experiment. KNO3 or NH4C1 were applied 27 days after planting at 0,16, 32, 64 and 128 kg N/ha. Nodulation and growth of both cultivars significantly increased when N was applied at low levels whereas specific N2-ase activity (SNA) slightly and insignificantly increased. Cv Crauford showed a greater positive response than cv. Clark. Higher rates of KNO3 and NH4C1 (128 kg N/ha) significantly depressed nodulation and SNA but slightly decreased the plant dry matter. Cv. Crauford was more tolerant to N fertilizers than cv. Clark. The decline in SNA was ascribed to increased nitrate reductase activity (NRA) and higher accumulation of nitrites in nodule cytosol. NRA and nitrate contents in nodules of cv. Clark were greater than that in cv. Crauford. Results showed that NH4 + is the preferred N source with occasional increases in nodule number and weight. This study provides an evidence for the nodulation and growth variability of soybean cultivars fertilized with different levels of N. The results also suggest that diminishing NRA could contribute to increased N2 fixation and the interaction between NO3- assimilation and N2 fixation is strongly dependent on the plant cultivar.