Cardiovascular disease (CVD) is a major cause of mortality and morbidity worldwide, often with heart failure as the terminal stage. Clinical studies have associated elevated levels of trimethylamine N-oxide (TMAO), a gut-derived metabolite, with adverse outcomes of CVD. As of today, TMAO's effects on cardiac structure and function are not well understood. In this study, both male and female TMAO-treated hearts showed functional deficits based on electrocardiography and echocardiography results. Immunohistochemistry results showed signs of hypertrophic cardiomyopathy in TMAO-treated male hearts while female TMAO-treated hearts showed signs of dilated cardiomyopathy. Neither TMAO group showed signs of fibrosis. Overproduction of reactive oxygen species was only observed in male TMAO-treated hearts. At the level of individual cardiomyocytes, significant delays in time to reach maximum contraction and dilation were only seen in TMAO-treated male hearts along with higher contractile force. Overall, TMAO-treated hearts show significant functional deficits with altered structure in a sex-specific way. Our study utilizes a variety of methods to comprehensively characterize features of TMAO-induced heart failure in both males and females which extends our current knowledge from human clinical associations. / Master of Science / Cardiovascular disease (CVD) is a major cause of mortality and morbidity worldwide, often with heart failure as the terminal stage. Clinical studies have associated elevated levels of trimethylamine N-oxide (TMAO), a compound derived from eggs, red meat and seafood, with adverse outcomes of CVD. As of today, TMAO's impact on the heart is not well understood. After supplementing mice with TMAO, we discovered deficiencies in heart function coupled with altered heart structure showing signs of hypertrophic cardiomyopathy in males and dilated cardiomyopathy in females. In-depth experiments suggest that TMAO-induced cell stress could be a potential underlying cause of previously mentioned changes but the specific mechanisms require further investigation. Overall, TMAO-treated hearts show significant functional deficits with altered structure in a sex-specific way. Our study utilizes a variety of methods to characterize features of TMAO-induced heart failure aiming to unravel relevant biological changes in both male and female mice which extends our knowledge from human clinical associations.