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This study was conducted to compare the cardiac effects of particulate matter (PM)- (SA-PM) and ozone(O 3 )-enhanced (SA-O 3 ) smog atmospheres in mice. Based on our previous findings of filtered diesel exhaust we hypothesized that SA-O 3 would cause greater cardiac dysfunction than SA-PM. Radiotelemetered mice were exposed to either SA-PM, SA-O 3 , or filtered air (FA) for 4 h. Heart rate (HR) and electrocardiogram were recorded continuously before, during and after exposure. Both SA-PM and SA-O 3 increased heart rate variability (HRV) but only SA-PM increased HR. Normalization of responses to total hydrocarbons, gas-only hydrocarbons and PM concentration were performed to assess the relative contribution of each phase given the compositional variability. Normalization to PM concentration revealed that SA-O 3 was more potent in increasing HRV, arrhythmogenesis, and causing ventilatory changes. However, there were no differences when the responses were normalized to total or gas-phase only hydrocarbons. Thus, this study demonstrates that a single exposure to smog causes cardiac effects in mice. Although the responses of SA-PM and SA-O 3 are similar, the latter is more potent in causing electrical disturbances and breathing changes potentially due to the effects of irritant gases, which should therefore be accounted for more rigorously in health assessments.