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The effects of chagasic sera containing an antibody (EVI antibody) on "non toxic" and "toxic" actions of ouabain on isolated rat atria suspended in different media, were explored. Ouabain failed to evoke any significantly positive contractile effect on atria beating in EVI Positive Human Sera (EVI(+)S), and only produced "toxic" actions (arrhythmias, contracture enhanced frequency, reduced inotropism). On the contrary, atria beating in Krebs-Ringer-Bicarbonate (KRB) or in Normal Human Sera (NHS) reacted to added ouabain with a classical dose-dependent positive inotropic effect. The threshold concentration of ouabain required to elicit the onset of "toxic" effects was higher in KRB than in EVI(+)S. Decreasing the extracellular Ca2+ concentration or treating the auricles with verapamil, facilitated positive inotropic influence of ouabain and attenuated the "toxic" effects observed on atria beating in EVI(+)S. In addition, the combination of (--)-propranolol plus verapamil enhanced the facilitatory influence of verapamil induced on the positive inotropic effect of ouabain and reduced the "toxic" influences evoked on atria exposed to EVI(+)S. These findings support the notion that the overall "toxic" responses to ouabain on cardiac tissue immersed in an EVI(+)S containing solution may be related to a combined effect of the EVI antibody activating beta-adrenoceptors as well as to a rise in the tissue Ca2+ content.