Chagasic (Ch) and non-chagasic immunoglobulin G fraction (NCh) (20µg.ml^-1) effects on cardiac performance of adult Wistar rat ventricles were studied with a novel approach by applying a microcalorimetric technique. Resting heat (Hr) was significantly decreased by Ch antibodies (Hr_Ch=4.8±0.9 mW.g^-1). While the Hr decrease can be associated to a diminished activity of the Na⁺/K⁺ pump, the magnitude of the effect (25% of control Hr) indicates that additional processes may also be affected. Ch antibodies induced an initial increase in developed pressure (P), which was associated with a decreased contractile economy. However, after 30min of Ch antibodies perfusion, P reached a significantly lower level (P_Ch=3.8±1.2 mN.mm^-2) without changes in active heat per beat (H'a). Consequently, H'a/P ratio increased indicating that energetic cost per unit of P was higher. On the other hand, P and H'a were both significantly and reversibly decreased by NCh antibodies (P_NCh=4.4±1.2 mN.mm^-2 and H'a_NCh=9.7±2.2 mJ.g^-1 respectively), but H'a/P ratio remained unaffected. According to these data, normal hearts exposed to Ch antibodies present a biphasic mechanical response: (a) an initial period of increased contractility (and decreased global muscle economy) which is consistent with antibodies with 1-adrenergic activity as the ones used in the present study, and (b) a decrease in P at 30min Ch antibodies perfusion which suggests that another Ca²⁺ related mechanism is compromised. These data contribute to redefine the role for antibody-mediated responses in the pathophysiology of chronic chagasic cardiomyopathy as agents of myocardial failure.