Sustained Akt activation induces cardiac hypertrophy (LVH), which may lead to heart failure. This study tested the hypothesis that Akt activation contributes to mitochondrial dysfunction in pathological LVH. Akt activation induced LVH and progressive repression of mitochondrial fatty acid oxidation (FAO) pathways. Preventing LVH by inhibiting mTOR failed to prevent the decline in mitochondrial function but glucose utilization was maintained. Akt activation represses expression of mitochondrial regulatory, FAO, and oxidative phosphorylation genes in vivo that correlate with the duration of Akt activation in part by reducing FOXO-mediated transcriptional activation of mitochondrial-targeted nuclear genes in concert with reduced signaling via PPARα/PGC-1α and other transcriptional regulators. In cultured myocytes Akt activation disrupted mitochondrial bioenergetics, which could be partially reversed by maintaining nuclear FOXO, but not by increasing PGC-1α. Thus, although short-term Akt activation may be cardioprotective during ischemia by reducing mitochondrial metabolism and increasing glycolysis, long-term Akt activation in the adult heart contributes to pathological LVH in part by reducing mitochondrial oxidative capacity. Three samples per group of 8-week-old wild-type or transgenic mice with cardiac-specific constitutive expression of an activated Akt (caAkt) in the heart at 8 weeks of age were used. Mice have been previously described in depth (Shioi T, McMullen JR, Kang PM, Douglas PS, Obata T, Franke TF, Cantley LC, Izumo S. 2002. Akt/protein kinase B promotes organ growth in transgenic mice. Mol. Cell. Biol. 22:2799-2809.). After hearts were removed total myocardial RNA was labeled and processed as described below for microarray analysis to detail the global changes in gene expression underlying development of heart failure in this mouse model.