Depressed parasympathetic activity has been proposed to be associated with an increased risk of sudden death. Parasympathetic effects (PE) on cardiac electrophysiology during exercise and recovery have not been studied in patients with left ventricular dysfunction. We performed non-invasive electrophysiologic studies (NI-EPS) and characterized the electrophysiological properties of the sinus node, atrioventricular (AV) node, and ventricle in subjects with depressed left ventricular ejection fraction (LVEF) and dual-chamber defibrillators. NI-EPS was performed during rest, exercise and recovery at baseline and after parasympathetic blockade with atropine to assess PE (the difference between parameter values in the 2 conditions). Ten subjects (9 men, age 60±9 years, LVEF=29±8%) completed the study. All NI-EPS parameters decreased during exercise, and trended toward rest values during recovery. PE at rest, during exercise, and during recovery, respectively, on sinus cycle length were 320±71 ms (p=0.0001), 105±60 ms (p=0.0003), and 155±82 ms (p=0.0002); on AV block cycle length 137 ±136 ms (p=0.09), 37±19 ms (p=0.002), and 61±39 ms (p=0.006); on AV interval 58±32 ms (p=0.035), 22±13 ms (p=0.002), and 36±20 ms (p=0.001); on ventricular effective refractory period 15.8±11.3 ms (p=0.02), 4.7±15.2 ms (p=0.38), and 6.8±15.5 ms (p=0.20); on QT interval 13±12 ms (p=0.13), 3±17 ms (p=0.6), and 20±23 ms (p=0.04). In conclusion, we describe for the first time the changes in cardiac electrophysiology and PE during rest, exercise and recovery in subjects with left ventricular dysfunction. PE are preserved in these patients. Thus, the role of autonomic changes in the pathophysiology of sudden death requires further exploration.