The cardiotoxicity of Coffee senna (Senna occidentalis) was investigated in sheep that were fed diets containing its seeds, which are recognized as the most poisonous part of such weed. Dianthrone, the main toxic component of S. occidentalis, is known to impair mitochondrial oxidative phosphorylation, leading to myofiber degeneration. In this study, fifteen ewes were fed 0%, 2% or 4% of seeds of S. occidentalis for 63 days. Non-specific markers of myocyte injury and electrocardiograms were undertaken at baseline, and at 14, 35, and 63 days after the animals were first fed the diets, while histopathology of heart samples was performed at the very end of the study. Our results showed an increase in serum AST and LDH over time, while CK-MB did not change significantly. Changes that could be ascribed to myocardial damage were not documented in the electrocardiograms. Cardiac histopathology demonstrated only mild-to-moderate vacuolar degeneration, myofiber edema and disarray, structural disorganization, and cellular necrosis. In conclusion, S. occidentalis caused myocardial fiber degeneration in a dose-dependent fashion, but the electrocardiogram was not able to identify these lesions non-invasively. Because the markers of myofiber injury used in this study lack specificity, they may not be used to support cardiac impairment objectively, despite some of them did change over time.