The adaptation of cardiovascular and thermoregulatory responses to ozone (O3) was examined by repeated exposure of electrocardiographic (ECG) electrode- and thermistor sensor-mplanted rats to O3 for 4 consecutive days (8hrs/day). Circadian rhythms of heart rate (HR) and abdominal core temperature (Tco) were disrupted on the first and second O3 exposure days in a concentration dependent manner. The 8-hr and 12-hr averaged values of HR and Tco during the exposure period and the dark post-exposure period, respectively, decreased significantly on the first and second exposure days. The decreased HR and Tco recovered to respective control values after small but significant rebound increases on the third and fourth days of O3 exposure. The adaptation of the extrapulmonary responses to O3 exposure was discussed in light of the previously reported time periods required to abolish the spontaneous breathing, biochemical, cellular, and morphological responses to O3.