Abstract: Pot experiment was carried out,using three spinach cultivars as test plants and at two N application rates(N 0.1 and 0.3 g/kg),to study the in vivo and the in vitro nitrate reductase activities(NRA),nitrate metabolic pool size(NMPS),storage pool size(NSPS) and the changes of the leave blade NRA response to excess nitrate.The obtained results showed that the in vivo NRA,the ratio of the in vovo to in vitro NRA,the NMPS and the ratio of NMPS to NSPS of blade are negatively related to the nitrate-N concentration in petiole.The percentage increase of blade NRA after cultivated by extra-source nitrate was positively related to the petiole nitrate-N concentration.Therefore,the in vivo NRA of blade and its influence,NMPS and nitrate-N distribution between the MPS and SPS of blade cell were the critical reasons for the difference of nitrate-N concentration in petioles over different spinach cultivars.