On guinea pig isolated hearts perfused according to the Langendorff technique, by Chenoweth's solution, and by various concentration of ZnCl2, surface E.C.G. was recorded for 65 min, after the equilibration period. Concentrations of zinc corresponding to plasma free and exchangeable fractions (1.5 microM) exerted only a nonstatistically significant sinus bradycardia. Bradycardia was prominent between the 25th and 35th min of perfusion, with higher concentrations of zinc (7.5 and 15 microM). In even higher (toxic) concentrations of zinc (30 microM), various types of arrhythmias were recorded, such as first degree AV block, idioventricular rhythms, atrial, and ventricular extrasystoles, and asystolia, evoked since the 15th min of perfusion. The above-mentioned findings in relation to low plasma zinc concentrations found in coronary patients and experimental myocardial infarction as well as myocardial injury related to metal fume fever, brought our attention to the involvement of this divalent cation to cardiac physiology and pathophysiology.