A number of field experiments were conducted to assess the role of combined application of nitrogen and sulfur to increase the seed and oil yield of nonnodulating soybean (Glycine max (L) Merr.) cv. PK-416 $(V_1)$ and cv. PK-1024 $(V_2)$. Six combinations of N and S in three replicates each were used for this purpose i.e. $0;S+23.5kg;N;ha^{-1}(T_1);;0;S+23.5+20kg;N ;ha^{-1}(T_2);;40;S+23.5kg;N;ha^{-1}(T_3);;40;S+23.5+20kg;N;ha^{-1}(T_4);;20+20;S+23.5kg;N;ha^{-1}(T_5);;20+20;S+23.5+20kg;N;ha^l(T_6)$. Nitrate reductase (NR) and ATP-sulphurylase activities in the leaves were measured at various growth stages as the two enzymes catalyze the rate limiting steps of the assimilatory pathways of nitrate and sulphate, respectively. The activities of these enzymes were strongly correlated with seed yield. The higher seed, oil and protein yields were achieved with the treatment $T_6$ in both the cultivars due to optimization of NR activity and ATP-sulphurylase activity, as these parameters were influenced by N and S assimilation. Any variation from this combination was observed to decrease the activity of these enzymes resulting in reductions in the seed, oil and protein yield of soybean.