Epidemiology studies suggest that exposure to air pollution increases the frequency of cardiac arrhythmias. A limitation of these studies is that it is difficult to link an increased risk of arrhythmias to a specific air pollutant. Animal exposure studies offer the opportunity to examine the effects of concentrated ambient fine particulate matter (PM), ultrafine PM, and copollutant gases separately. Male Fischer 344 rats, aged 18 mo, with implanted electrocardiograph (ECG) transmitters were used to determine the effects of PM on the frequency of arrhythmias. We found that old F344 rats had many spontaneous arrhythmias. An arrhythmia classification system was developed to quantify arrhythmia frequency. Arrhythmias were broadly grouped into two categories: premature beats and delayed beats. The rats were exposed to concentrated ambient PM (CAPS) or air for 4 h. The rats were exposed twice with a crossover design so each rat could serve as its own control. The CAPS concentrations were 160 microg/m(3) and 200 microg/m(3) for the first and second exposures, respectively. There was a significant increase in the frequency of irregular and delayed beats after exposure to CAPS. The same rats were subsequently exposed to laboratory-generated ultrafine carbon particles, to SO(2), or to air with a repeated crossover design. In these experiments there was no significant change in the frequency of any category of spontaneous arrhythmia following exposure to ultrafine carbon or SO(2). Thus, this study adds supporting evidence that acute exposure to elevated levels of ambient PM increases the frequency of cardiac arrhythmias.