SummaryStomatal responses to air pollutants are complex, varying among species and with concentration, environmental conditions and age. In general, short-term exposure to sulphur dioxide (SO 2 ) promotes stomatal opening, whereas longer-term exposure can cause partial stomatal closure. By contrast, the effects of oxides of nitrogen (NO x ) are often small or insignificant. The effects of ozone, and oxidative stress, are equally complex. Short-term exposure to ozone stimulates a rapid reduction in stomatal aperture, whilst longer-term exposure causes stomatal responses to become sluggish. The response of stomata to abscisic acid (ABA) has been shown to be slower in plants exposed to a combination of SO 2 and NO 2 suggesting an adverse effect on guard cell ABA signal transduction. In addition, ozone causes a reduction in stomatal closure under drought conditions. There is an increasing body of evidence to suggest that air pollutants and oxidative stresses can have a marked effect on the Ca 2+ homeostasis of guard cells and the intracellular machinery responsible for stomatal movements. Here we discuss the effects of air pollutants on stomatal responses and their possible effects on Ca 2+ based signalling in guard cells focusing on the effects of ozone and oxidative stress. [Ca 2+ ] cyt , cytosolic free calcium; GST, glutathione S -transferase; K in , plasma membrane inward-rectifying K + channel; NO x , oxides of nitrogen. Abbreviations© New Phytologist (2002) 153 : 441-447