Calcium is well established as a key signalling molecule in plant and animals systems. Stimuli such as heat, cold, touch and ozone (O3) cause an increase in cytosolic calcium concentration [Ca2+]cyt in plants. O3 is a reactive oxygen species (ROS) and acts as a pollutant in the troposphere, whereas stratospheric ozone protects the earth from damaging ultra violet radiation. ROS are able to cause damage to plant cell components, effects include lipid peroxidation, reduced transpiration, a decrease in photosynthesis and a reduction in productivity. We have been studying the early molecular events in the response of Arabidopsis thaliana to O3 focusing on the role of calcium ions. Arabidopsis thaliana transformed with the photoprotein apoaequorin have been used to measure changes in [Ca2+]cyt during fumigation with O3. Elevations in [Ca2+]cyt occur in a dose dependent manner and a ‘calcium signature’ characteristic of O3 exposure is seen. Increases in antioxidant defences also occur following O3 exposure, however the signal transduction pathways leading from the perception of O3 stress and adaptation to the stress are poorly understood. The ozone induced increase in [Ca2+]cyt has been implicated in the induction of the antioxidant gene glutathione-S-transferase tau 5 (GSTU5) (Clayton et al. (1999) The Plant Journal 17(5) 575-579) . This study provides a continuation of this work. A pharmacological analysis of the calcium signature in response to O3 has been undertaken. Two inhibitors of calcium release pathways, lanthanum, a plasma membrane channel blocker and ruthenium red, an intracellular calcium channel blocker, have been used, as has the calcium chelator EGTA. The calcium response following treatment with inhibitors of other signalling molecules and components of signal transduction pathways has also been investigated. These include the cADPR inhibitor nicotinamide and the phospholipase C inhibitor 2-APB. The research has been extended to include an analysis of the effect of these inhibitors on GSTU5 expression in response to ozone. In addition a genetic analysis of the ozone-calcium signature has been performed using the calcium transport mutant ACA3 transformed with apoaequorin. This has a plasma membrane autoinihibited calcium ATPase gene knocked out. Theere is little difference between the mutant and wild type response to O3, suggesting redundancy within the system or that an alternative source of calcium is involved in the ozone response