The electrical properties of the mammalian heart undergo many complex transitions In normal and diseased states(1-7). It has been proposed that the normal heartbeat may display complex nonlinear dynamics, including deterministic chaos(8,9), and that such cardiac chaos may be a useful physiological marker for the diagnosis(10-12) and management(13,14) of certain heart trouble, However, it is not clear whether the heartbeat series of healthy and diseased hearts are chaotic or stochastic(15-17), or whether cardiac chaos represents normal or abnormal behaviour’s. Here we have used a highly sensitive technique, which is robust to random noise, to detect chaos(19). We analysed the electrocardiograms from a group of healthy subjects and those with severe congestive heart failure (CHF), a clinical condition associated with a high risk of sudden death. The short-term variations of beat-to-beat interval exhibited strongly and consistently chaotic behaviour in all healthy subjects, but were frequently interrupted by periods of seemingly non-chaotic fluctuations in patients with CHE Chaotic dynamics in the CHF data, even when discernible, exhibited a high degree of random variability over time, suggesting a weaker form of chaos. These findings suggest that cardiac chaos is prevalent in healthy heart, and a decrease in such chaos may be indicative of CHF.