THE EFFECTS of treatment with 100mM NaCl on photosynthetic activity, protein and proline contents , activities of key nitrogen metabolism enzymes, nitrate reductase (NR) and nitrite reductase (NiR), ammonia assimilating enzymes: Glutamine synthetase (GS), glutamate synthase (GOGAT) and glutamate dehydrogenase (NADH-GDH) activities and finally ammonia and nitrate contents were investigated in 40-day old leaves of tomato plants. There was a decline in total protein (TP) and insoluble protein (InsP) fractions accompanied with a significant increase in proline and soluble protein contents in response to NaCl – stress. Under salinized conditions, there was a significant inhibition in all tested leaf gas exchange parameters, stomatal conductance (gs), internal CO2 concentration (Ci) and hence, CO2 assimilation (A). Also, a significant inhibition of NR & NiR enzymes and a strongly decrease in nitrate content were observed. In contrast, ammonia assimilating enzymes (GS, GOGAT, NADH-GDH and NAD-GDH) activities were obviously increased in NaCl – salinized tomato leaves, this accompanied with a significant increase in ammonia content. Soaking tomato seeds in 10μM sodium nitroprusside (SNP) for 8h were elevated-to some extent- all the studied parameters and there was an improvement in TP, proline, all gas exchange parameters, NR & NiR activities and nitrate content. While ammonia assimilating enzymes (GS, GOGAT, NADH-GDH and NAD-GDH) activities and ammonia content were significantly decreased compared to NaCl - salinized tomato leaves.