Recent epidemiological studies have reported a positive association between levels of ambient particulate matter ( PM) and daily morbidity and mortality due to respiratory or cardiovascular causes; however, toxicological evidence supporting these ndings is limited. The present study compared cardiac and thermoregulatory responses to intratracheal instillations of residual oil y ash ( ROFA) in normal and cardiopulmonary-compromised male Sprague-Dawley rats. Animals ( n = 64) were implanted with radiotelemetry transmitters capable of continuously monitoring heart rate, core body temperature, and electrocardiographic waveforms. Comparisons of ROFA toxicity were conducted between ( 1) healthy rats and rats with cardiopulmonary stress or disease, including ( 2) rats exposed to an ambient temperature of 10°C, ( 3) rats preexposed to ozone to induce pulmonary in ammation, and ( 4) rats pretreated with monocrotaline ( MCT) to induce pulmonary hypertension and vasculitis. Animals from each regimen were instilled with 1 of 4 doses of ROFA ( 0, 0.25, 1.0, 2.5 mg) , and telemetry data were acquired for 96 h following ROFA instillation. Doserelated hypothermia and bradycardia were observed in healthy animals following exposure to ROFA; the magnitude and duration of these responses were potentiated in all compromised models. Delayed hypothermic and bradycardic responses occurred in healthy animals receiving 2.5 mg ROFA up to 48 h following instillation. These delayed responses were exacerbated in the MCT-and 10°C-exposure models, but attenuated in the O 3 -preexposed group. Additional observed effects of ROFA included induction of cardiac arrhythmiasThe authors thank Dr. Urmila P. Kodavanti, Julie P. Nolan, and James A. Raub for their thoughtful reviews of this article. Further gratitude is extended to