Nitrate reductase is a key enzyme in the overall process of nitrate assimilation by plants. A full-length cDNA clone encoding nitrate reductase (NR; EC 1.6.6.1) was isolated from cucumber (Cucumis sativus L.) by RT-PCR and RACE techniques. The NR of cucumber (CsNR), a full-length cDNA sequence of 3032 bp contains an open reading frame of 2748 bp encoding 915 amino acids. The deduced 915 amino acid sequence showed high identities with NR from other plants. Quantitative real-time PCR analysis indicated that CsNR expression was different in root, stem, leaf, flower and mature fruit tissues. CsNR transcript level and nitrate reductase activity (NRA) was down-regulated and the change in NO(3) (-) concentration showed a negative trend with NRA in leaves when subjected to the 182 mM NO(3) (-) treatment. However, the CsNR transcript level was up-regulated in roots by 182 mM NO(3) (-) treatment. Furthermore, NRA in roots lagged behind CsNR expression and there was no obvious lag of NRA in leaves. This study found that in roots, there was no obvious relationship between NRA and NO(3) (-) content. These results indicated that NRA was not only controlled by the level of CsNR mRNA and there was an obvious negative relationship between NO(3) (-) content and NRA in leaves but not in roots.