Objectives: Epidemiological studies of air pollution on cardiovascular health show associations of cardiac mortality and admissions with exposure to nitrogen dioxide (NO2) at low concentrations. These associations could be causal or NO2 could be acting as a surrogate measure for another air pollutant, most likely ultrafine particles. No studies of cardiac susceptibility to acute exposure to NO2 have been undertaken. Methods Randomised controlled exposures to NO2(400 ppb for 1 h) and air in subjects with coronary heart disease and impaired left ventricular systolic function not taking β adrenoceptor blocking drugs. Results: There were no significant changes in heart rate, blood pressure, leucocyte coping capacity or any heart rate variability measure following NO2 exposure compared with air. Conclusion: These findings suggest that NO2 does not affect heart rate variability at these concentrations (which are high for urban background levels) and in the absence of other pollutants. While a synergistic effect has not been ruled out, these data lend support to the idea that the epidemiological data associating cardiac outcomes with NO2 are more likely due to an associated pollutant rather than NO2 itself.