Abstract: Particulate matter air pollution is associated with increased cardiovascular mortality. The present study examined the cardiac effects of diesel exhaust exposure in spontaneously hypertensive rats. These rats (4 mo old, n=6 males and 4–6 females/concentration) were exposed to one of five diesel exhaust levels (0, 30, 100, 300, and 1000 μg particles/m3) for 6 h per day for 7 d. Electrocardiographic measurements were obtained by radiotelemetry beginning 3 d prior to exposure and ending 4d after exposure cessation. Control rats displayed a reduced daytime heart rate from the beginning of the protocol, whereas exposed rats maintained a significantly elevated heart rate throughout the exposure. Daytime heart rate values for male control rats averaged 265±5 beats/min (mean±standard error [SE]), whereas values for exposed rats averaged 290±7 beats/min. This difference persisted during the evenings of the exposure period but was not observed at any time during the preex-posure or postexposure periods. The PQ interval, an index of atrioventricular node sensitivity, was significantly prolonged among exposed animals in a concentration-dependent manner. Increased heart rate with prolongation of the PQ interval may represent a substrate for ventricular arrhythmias. These results concur with previous reports suggesting that realistic exposure concentrations of air pollution affect the pacemaking system of rats.