Myocardial depressant factors have long been recognized to be present in burn serum (BS) and contribute to burn-generated cardiac contractile dysfunction. However, much of the cellular and molecular mechanism for its role in the development of the cardiac deficiency remains unknown. In this study, we investigated the effect of BS on myocardial contractility and Ca handling in single rat cardiomyocytes. The results revealed that BS (5% by volume) bidirectionally regulated cardiac excitation-contraction (EC) coupling. The action potential-elicited Ca transient and cell shortening were increased by 28.0% ± 9.7% and 34.7% ± 12.5% within 20 min after BS stimulation (the upregulation phase), but decreased by 20.5% ± 6.8% and 32.3% ± 5.1% at 60 min after BS stimulation (the downregulation phase). There was a 32.0% ± 5.8% reduction in sarcoplasmic reticulum (SR) Ca content at the downregulation phase, whereas no alteration was detected at the upregulation phase. The incidences of spontaneous Ca sparks and Ca waves were significantly increased after BS stimulation, no matter at the upregulation or downregulation phase. The hyperactive Ca sparks and Ca waves could be completely abolished by antioxidative treatment (vitamin A, 0.2 mM; and vitamin E, 1 mM) and partially reversed by NOS inhibitor L-NAME (100 μM), but not by blocking Ca influx with nifedipine (1 μM). With the normalization of Ca sparks, BS-induced alterations of action potential-elicited Ca transient and contractility were prevented by antioxidative therapy. Taken together, we propose that BS-associated bidirectional regulation of EC coupling is attributed largely to oxidative stress-induced hyperactivity of ryanodine receptors, increasing EC coupling through enhancing intracellular Ca release initially, but subsequently decreasing EC coupling by partially depleting SR Ca content through enhancement of Ca spark-mediated SR leak.