One of the properties of systemic acquired resistance in plants is its concomitance with the biochemical changes including enhancement of activities of defense-related enzymes. In this study, the effects of Acibenzolar-S-methyl (ACI) on the some of defense responses of the cucumber plants (healthy or inoculated with spore suspension of Podosphaerafusca, the causal agent of cucumber powdery mildew) were surveyed via in vivo tests. Changes of defense responses in ACI-treated cucumber plants, inoculated with pathogen or not, were studied and compared with those of non-treated control plants. Results indicated that specific activity of peroxidase increased significantly in treated plants. Increase in enzyme activity was higher in pathogen-inoculated than non-inoculated plants, thus pathogen attack stress to plant plays a role in enhancement of enzyme activity. Specific activity of phenylalanine ammonialyase showed no changes in ACI-treated non-inoculated plants, but in inoculated plants it Increased due to interaction between ACI treatment and pathogen attack stress. Specific activity of chitinase increased in both inoculated and non-inoculated ACI-treated plants at 24 hours after treatment onwards, and pathogen attack stress did not affect it. Phenolic content of ACI-treated plant tissues, despite of small fluctuations, did not show any definite pattern of changes.