Rationale: Peroxisome proliferator-activated receptors (PPARs) (α, γ, and δ/β) are nuclear hormone receptors and ligand-activated transcription factors that serve as key determinants of myocardial fatty acid metabolism. Long-term cardiomyocyte-restricted PPARS deficiency in mice leads to depressed myocardial fatty acid oxidation, bioenergetics, and premature death with lipotoxic cardiomyopathy. Objective: To explore the essential role of PPARδ in the adult heart. Methods and Results: We investigated the consequences of inducible short-term PPARδ knockout in the adult mouse heart. In addition to a substantial transcriptional downregulation of lipid metabolic proteins, short-term PPARS knockout in the adult mouse heart attenuated cardiac expression of both Cu/Zn superoxide dismutase and manganese superoxide dismutase, leading to increased oxidative damage to the heart. Moreover, expression of key mitochondrial biogenesis determinants such as PPARγ coactivator-1 were substantially decreased in the short-term PPARS deficient heart, concomitant with a decreased mitochondrial DNA copy number. Rates of palmitate and glucose oxidation were markedly depressed in cardiomyocytes of PPARδ knockout hearts. Consequently, PPARδ deficiency in the adult heart led to depressed cardiac performance and cardiac hypertrophy. Conclusions: PPARδ is an essential regulator of cardiac mitochondrial protection and biogenesis and PPARδ activation can be a potential therapeutic target for cardiac disorders. © 2010 American Heart Association, Inc.