Introduction: Growing body of evidence has proven the use of natural polyphenolic compounds as nutraceuticals for cardiovascular diseases. Polyphenols have been shown to protect against cardiovascular-related diseases such hypertension, diabetes and atherosclerosis. Roselle (Hibiscus sabdariffa Linn.) is local natural source of bioactive polyphenols which was shown to exert protective effects on cardiovascular system e.g. vasodilator in aortic rings and antihypertensive via ion channels in the heart. We have recently shown in our laboratory that Roselle extract attenuate nicotine-induced cardiac injury in rats1. Nevertheless, effects of Roselle polyphenols on cardiac physiology are unresolved. The aim of this study was to investigate the effect of Roselle polypehnols (RP) on healthy Langendorff-perfused rat hearts. Method: Adult Sprague-Dawley rat (n=12, 14 weeks old) hearts were perfused in constant flow Langendorff mode at 10 ml/min (baseline perfusion pressure 77mmHg, LVDP 106mmHg, heart rate 241bpm). The hearts were validated by positive inotrope isoprenaline (1 nM) and negative inotrope sodium nitrite (100 mM). RP (125-2000 mg/mL) was given via Krebs-Henseleit buffer perfusion to the heart and changes in cardiac function were compared against time-matched vehicle controls. Values are given as means ± S.E.M., compared by one-way ANOVA and t test. Results: HPLC profiling of RP extract revealed twelve flavonoids and seven phenolic acids confirming its polyphenolic content. Direct perfusion of RP suppressed rat heart systolic function as shown by lowered LVDP and maximal velocity of contraction (+dP/dtmax). Besides, RP also reduced heart rate while simultaneously increasing maximal velocity of relaxation (-dp/dtmin). RP significantly increase cardiac injury markers troponin T and lactate dehydrogenase (LDH) in coronary effluent at 1000 and 2000 mg/ml only. To further investigate involvement of calcium channels, inotropic responses were firstly enhanced by agonists of different receptors: L-type Ca2+ channel ((±)-Bay K 8644), ryanodine receptor (4-chloro-m-cresol), b-adrenergic receptor (isoproterenol), and also a SERCA blocker (thapsigargin) prior to administration of RP, which blunted all inotropic responses. Conclusion: Altogether, RP negatively regulates cardiac contractile function (inotropism), heart rate (chronotropism) but positively regulates ventricle relaxation (lusitropism). Cardiac actions of RP may involve at least in part, modulation of calcium channels but further studies are warranted to strengthen this observation.