Abstract Body: Metabolic Syndrome (MetS) affects ~35% of the adult population in the US and increases the risk for cardiovascular complications and all-cause mortality. Cardiac autonomic imbalance occurs with MetS, raising the possibility that altered sympathetic tone modulates cardiac and myocyte function, contributing to the manifestation of the cardiometabolic phenotype. To address this possibility, MetS was induced in C57Bl/6 mice by chronic administration (>4 months) of a Western diet (WesD). Mice on regular chow were used as control (Ctrl). Using ECG recordings, male and female WesD mice had reduced RR interval duration (i.e., increased heart rate) and reduced heart rate variability, with respect to Ctrl animals, consistent with sympathoexcitation. Importantly, inhibition of β-adrenergic receptor (β-AR) with propranolol eliminated differences in RR interval duration between the two groups of mice. By echocardiography, systolic and diastolic function assessed, respectively, by ejection fraction (EF) and the isovolumic relaxation time (IVRT), were comparable between the two groups of mice at baseline. However, acute β-AR blockade resulted in reduced EF (-9%) and prolonged IVRT (+18%) in WesD mice, with respect to Ctrl animals. To evaluate the influence of sympathoexcitation on left ventricular cardiomyocyte function, cells were obtained from hearts of wild-type mice and animals lacking β1- and β2-ARs (βAR-KO) maintained on control chow or Western diet. Functionally, left ventricular myocytes from WesD wild-type mice had enhanced fractional cell shortening (+34%) and faster relaxation (+6%), with respect to cells from Ctrl wild-type mice. In contrast, myocytes from βAR-KO mice on Ctrl and WesD had comparable cell shortening and kinetics of relaxation. Collectively, these findings indicate that metabolic syndrome induces activation of β-AR signaling in the heart concealing, at least in part, defects of cardiac function.