Salicylic acid (SA), a ubiquitous phenolic phytohormone, is involved in many plant physiological processes including stomatal movement. We analysed SA-induced stomatal closure, production of reactive oxygen species (ROS) and nitric oxide (NO), cytosolic calcium ion ([Ca2+]cyt) oscillations and inward-rectifying potassium (K+in) channel activity in Arabidopsis. SA-induced stomatal closure was inhibited by pre-treatment with catalase (CAT) and superoxide dismutase (SOD), suggesting the involvement of extracellular ROS. A peroxidase inhibitor, SHAM (salicylhydroxamic acid) completely abolished SA-induced stomatal closure whereas neither an inhibitor of NADPH oxidase (DPI) nor atrbohD atrbohF mutation impairs SA-induced stomatal closures. 3,3′-Diaminobenzidine (DAB) and nitroblue tetrazolium (NBT) stainings demonstrated that SA induced H2O2 and O2– production. Guard cell ROS accumulation was significantly increased by SA, but that ROS was suppressed by exogenous CAT, SOD and SHAM. NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) suppressed the SA-induced stomatal closure but did not suppress guard cell ROS accumulation whereas SHAM suppressed SA-induced NO production. SA failed to induce [Ca2+]cyt oscillations in guard cells whereas K+in channel activity was suppressed by SA. These results indicate that SA induces stomatal closure accompanied with extracellular ROS production mediated by SHAM-sensitive peroxidase, intracellular ROS accumulation and K+in channel inactivation.