The mechanism of stomatal closing by salicylic acid (SA) has been investigated. The addition of 1 mM SA to fully spened stomata resulted in a significant reduction of 75% in stomatal aperture. Stomata in the treatment of SA with EGTA closed as observed in the treatment of SA. However, the addition of catalase with SA completely inhibited stomatal closing. Stomatal closing induced by SA was also reduced by Ca2+. To understand the relation between stomatal closing by SA and catalase activity, the effect of SA on catalse activity and the effect of AT (catalase inhibitor) on stomatal closing was investigated. SA inhibited 32% of catalase activity. Stomata in isolated epidermis floated on an incubation solution containing 0.1 mM AT closed from 9.6 ${mu}{ extrm}{m}$ to 3.2 ${mu}{ extrm}{m}$ after 1 hour. SA stimulated K+ efflux as much as the twice of the control in isolated strips. SA inhibited 53% of photosynthetic activity at the light intensity of 1000 $mu$mole m-2 S-1 on SA infiltrated leaves. A similar result was found onstomatal conductance in SA infiltrated leaves. These results indicate that SA inhibit catalase activity and increase the concentration of H2O2 in guard cell cytoplasm. H2O2 oxidize the plasma membrane and increase the membrane permeability of K+. The mass efflux of K+ induce the loss of turgor pressure and lead to stomatal closing. The inhibition of photoxynthetic activity by SA suggests that stomatal closing by SA is also related with the decrease of photosynthetic activity.