BACKGROUND Among the general population, mental stress seems to depress heart rate variability (HRV), and physical activity level seems to be positively associated with HRV. However, the extent to which these relationships exist among patients with ischemic heart disease is not clear. Therefore, this study investigated the association between level of physical activity and HRV among patients with ischemic heart disease during conditions of paced breathing and the Stroop Color-Word Conflict Test (Stroop). METHODS Forty-two patients with ischemic heart disease were assigned to groups based on their documented volume of aerobic activity: Low = 1.008-2.646 kJ/wk; Mod = 3.024-3.864 kJ/wk; and High = 4.284-7.560 kJ/wk. These groups did not differ in age, body mass index, or hostility as determined by the Minnesota Multiphasic Personality Inventory. Time and frequency domain measures of HRV were derived from electrocardiograph data obtained during 5 minutes of paced respiratory control and 5 minutes of performing the Stroop Test. RESULTS There was a main effect of activity level (P < 0.05) on the standard deviation of R-wave to R-wave intervals (SDNN), total spectral power, and high-frequency power such that these dependent variables were greater in the High group than in the Low or Mod group. Furthermore, there was a main effect of the test condition on SDNN and total power, both of which were lower during the Stroop Test as compared to paced respiratory control. CONCLUSIONS The results indicate that, among patients with ischemic heart disease, a high level of physical activity is associated with higher HRV, but is not related to stress reactivity as measured by HRV.