Altered cardiac autonomic balance has been shown to be a predictor of adverse cardiac events in patients with cardiovascular decease (CAD). Alteration of the sympatho-vagal balance of heart regulation may be more evident during periods of mental stress. The high frequency (HF) component of heart rate variability (HRV) is a marker of the parasympathetic regulation of the heart. The meaning of the low frequency (LF) component of HRV is controversial, but it may represent the sympathetic modulation of heart regulation. Plasma catecholamine levels (epinephrine and norepinephrine) are markers of sympathetic nervous system action. Increases in hemodynamic (blood pressure and heart rate) responses to stress are predominantly regulated by the sympathetic branch of the autonomic nervous system (ANS). The relationships among the frequency domains of HRV, hemodynamics, and catecholamine level were examined at rest and during speech mental stress in 147 CAD patients from the Psychophysiological Investigations of Myocardial Ischemia (PIMI) study. At rest, there were no significant associations between HRV measures, hemodynamic responses, and plasma catecholamine levels. However, at peak stress levels, analyses of variance revealed a significant inverse relation between HF levels and epinephrine (Epi), (univariate F (3, 132) = 2.9; p= .037); and a significant inverse relation between LF and catecholamine levels, (multivariate F (6, 258) = 3.7; p =.001). At peak stress levels, HF levels were inversely related to systolic blood pressure (SBP) and heart rate (HR) (F SBP (3, 141) = 3.9; p =0.01; and F HR (3, 141) = 7.2; p < 0.001). At peak stress levels LF was inversely related to SBP and HR, (F SBP (3, 140) = 8.9; p < 0.001; and F HR (3, 140) = 4.0; p = 0.009). These findings suggest that associations of the vagal components (HF) of HRV with sympathetic markers in CAD patients are only revealed under conditions of mental stress. In addition, based on the data,