Electrocardiogram and arterial blood pressure of elastase-treated emphysematous rats (E rats) and saline-treated control rats (S rats) were recorded continuously during exposure to either 1 ppm ozone (O3) for 3 hr or 0.5 ppm O3 for 6 hr. The heart rates (HRs) of both groups decreased to about 50 and 65% of the initial levels at the end of 1 ppm and 0.5 ppm O3 exposure, respectively. Mean arterial blood pressures (MAPs) also decreased to about 76 and 82%, respectively. There was no significant difference in these responses between E and S rats, although the levels of HRs and MAPs of the E rats were always a little lower than those of the S rats. Another group of E and S rats was continuously exposed to 0.2 ppm O3 for 4 weeks. The HRs of both E and S groups decreased to about 81 and 88% of the initial levels on the first day, respectively, although they recovered completely by the third day. No significant difference in the variation of HRs during exposure was noted between E and S rats. However, the HR responses of these rats to a challenge exposure of 0.8 ppm O3 for 1.5 hr appeared to be different. That is, S rats were more tolerant of the challenge exposure to O3 for 1.5 hr than the E rats.