Nicotine induces a biphasic response in isolated beating-heart preparations. Nicotine at concentrations of 1 μM (left panel) and 10 μM (right panel) was perfused during 5 min. The bipolar 400 Hz acquisition rate EKG records indicated that 1 μM nicotine increased the heart rate of chagasic hearts (open circles), while decreasing it in healthy hearts (filled circles). 10 μM nicotine induced a biphasic effect (i.e., transiently bradychardia followed by sustained tachycardia) on both healthy and chagasic animals’ heart rate. * p < 0.05 indicated statistically significant difference compared to pre-drug basal rate.