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Fluctuations in heart rate above 0.03 Hz reflect autonomic modulation of sinoatrial node activity. To assess the dynamics of autonomic nervous activity during and immediately after exercise, we determined the power spectrum of heart rate and respiratory fluctuations in 43 normal subjects without known cardiac disease, 8 patients with severe congestive heart failure, and 6 patients status-post cardiac transplantation before, during, and after graded-work load exercise on a cycle ergometer. Before exercise, heart rate fluctuations (spectral power) at both high (0.15-0.80 Hz) and low (0.03-0.15 Hz) frequencies were significantly higher in normal subjects than in either heart failure or transplant patients but were not different between the two groups with heart disease. During exercise, heart rate power at all frequencies rapidly and progressively decreased in normal subjects, until at peak exercise it was not different from …