Studies were carried out on influences of replacement of 25% NO₃^--N with NH₄⁺-N in nutrient solution (enhanced ammonium nutrition) on tomato growth and activity of such key enzymes in N assimilation and organic acids as nitrate reductase (NR), glutamine synthetase(GS) and phosphoenolpyruvate carboxylase (PEPcase) in tomato leaves and fruits sampled at different developmental stages. The results showed that the replacement of 25% NO₃^--N with NH₄⁺-N led to a significant increase in fruit fresh weight, NH₄⁺-N content in both leaves and fruits and soluble protein content in fruits, while led to decrease signifi cantly in NO₃^--N content in tomato leaves. Meanwhile, significant enhancement of the activity of PEPcase in both leaves and fruits and of GS in leaves were observed but not much change in GS activity in tomato fruits was. NR activity was of ten observed to decrease dramatically in both fruit and leaves in enhanced ammonium treatment.