W e sought to delineate the molecular regulatory events involved in the energy substrate preference switch from fatty acids to glucose during cardiac hypertrophic growth. α₁-adrenergic agonist–induced hypertrophy of cardiac myocytes in culture resulted in a significant decrease in palmitate oxidation rates and a reduction in the expression of the gene encoding muscle carnitine palmitoyltransferase I (M-CPT I), an enzyme involved in mitochondrial fatty acid uptake. Cardiac myocyte transfection studies demonstrated that M-CPT I promoter activity is repressed during cardiac myocyte hypertrophic growth, an effect that mapped to a peroxisome proliferator–activated receptor-α (PPARα) response element. Ventricular pressure overload studies in mice, together with PPARα overexpression studies in cardiac myocytes, demonstrated that, during hypertrophic growth, cardiac PPARα gene expression falls and its activity is altered at the posttranscriptional level via the extracellular signal–regulated kinase mitogen-activated protein kinase pathway. Hypertrophied myocytes exhibited reduced capacity for cellular lipid homeostasis, as evidenced by intracellular fat accumulation in response to oleate loading. These results indicate that during cardiac hypertrophic growth, PPARα is deactivated at several levels, leading to diminished capacity for myocardial lipid and energy homeostasis.