Salt stress induced an increase in endogenous carbon monoxide(CO) production and the activity of the CO syntheticenzyme haem oxygenase (HO) in wheat seedling roots. Inaddition, a 50% CO aqueous solution, applied daily, notonly resulted in the enhancement of CO release, but led toa significant reversal in dry weight (DW) and water losscaused by 150 mM NaCl treatment, which was mimicked bythe application of two nitric oxide (NO) donors sodiumnitroprusside (SNP) and diethylenetriamine NO adduct(DETA/NO). Further analyses showed that CO, as well asSNP, apparently up-regulated H+-pump and antioxidantenzyme activities or related transcripts, thus resulting inthe increase of K/Na ratio and the alleviation of oxidativedamage. Whereas, the CO/NO scavenger haemoglobin(Hb), NO scavenger or synthetic inhibitor methylene blue(MB) or NG-nitro-L-arginine methyl ester hydrochloride(L-NAME) differentially blocked these effects. Furthermore,CO was able to mimic the effect of SNP by stronglyincreasing NO release in the root tips, whereas theCO-induced NO signal was quenched by the addition ofL-NAME or cPTIO, the specific scavenger of NO. Theresults suggested that CO might confer an increased toleranceto salinity stress by maintaining ion homeostasis andenhancing antioxidant system parameters in wheat seedlingroots, both of which were partially mediated by NO signal.