Abstract: Changes of endogenous polyamines and hormone under different NO3- stress were studied in cucumber seedlings growing by solution culture method. Results showed that, after treating with 56 and 98 mmol/L NO3- for 7 days, zeatin riboside (ZR), gibberellic acid (GA)and auxin (IAA) in cucumber roots were increased significantly, but abscisic acid (ABA) was decreased, which suggested that activating enzymes were related to hormone synthesis and caused the growth and development of cucumber seedlings. However, with the same treatment, ZR and GA were decreased, but ABA was increased in cucumber leaves, which suggested that the endogenous hormone in roots was the major cause for the growth of cucumber seedlings. With the treatment of 140 mmol/L NO3-, ZR, GA, IAA and ABA in leaves were significantly higher than control, but in roots, ZR, GA and IAA was decreased, while ABA was increased, thus, it might stimulate senescence of cucumber seedlings. With the treatment of NO3- concentration between 56 and 98 mmol/L, spermine (Spm) and spermidine (Spd) in leaves and Spm content in roots were increased with concentration of NO3-, but decreased when concentration reached 140 mmol/L and higher. Under 140 mmol/L NO3- treatment, putrescine (Put) in leaves were significantly lower than control，but the ratio of (Spd+Spm)/Put in both leaves and roots were remarkably higher than control. Thus, results suggested that changes and interaction of endogenous hormone (including polyamines) might be responsible for NO3- tolerance in cucumber seedlings.