Abstract: The effects of simulated acid rain on important N-metabolism enzymes and correlated substances were studied by treating spring and summer leaves of Eucommia ulmoides to simulated rain with varying pH levels. In both seasons it was found that below a threshold value of pH the activities of nitrate reductase (NR), glutamine synthetase (GS), glutamate dehydrogenase (GDH) and glutamic-pyruvic transaminase (GPT) declined with decreasing pH, though the rate of decrease of these enzymes were higher in the summer. It was also found that soluble protein and total nitrogen content decreased with decreasing pH of acid rain. Total free amino acid content was negatively correlated with pH in both seasons. We conclude from these results that simulated acid rain had significant effects on N-metabolism of Eucommia ulmoides leaves.