Abstract: The effects of nitric oxide (NO) donor on the changes of hydrogen peroxide （H2O2） content and its scavenging enzymes activities were surveyed in detached leaves of wheat (Triticum aestivum L.). Treatments with different concentrations of sodium nitroprusside (1 mmol/L and 5 mmol/L SNP), a nitric oxide donor in vivo, enhanced H2O2 levels significantly within 30 minutes of incubation. Meanwhile, guaiacol peroxidase (POD) activities were dramatically inhibite d, and the activities of catalase (CAT) displayed slight decrease. However, prolonging the treatment to 240 minutes gradually reduced the content of H2O2, which might be due to the restoration of CAT activities rather than consistent inhibition of POD. In vitro experiments also indicated that the inhibition of wheat POD in crude extracts by varying concentrations of SNP appeared to be irreversible in comparison with the reversible inhibition of CAT. Therefore, NO played a role in regulating H2O2 levels in detached wheat leave in vivoby different inhibiting modes of POD and CAT.