The roles of CDK4 in the cell cycle have been extensively studied, but less is known about the mechanisms underlying the metabolic regulation by CDK4. Here, we report that CDK4 promotes anaerobic glycolysis and represses fatty acid oxidation in mouse embryonic fibroblasts (MEFs) by targeting the AMP- activated protein kinase (AMPK). We also show that fatty acid oxidation (FAO) is specifically induced by AMPK complexes containing the alpha 2 subunit. Moreover, we report that CDK4 represses FAO through direct phosphorylation and inhibition of AMPK alpha 2. The expression of non- phosphorylatable AMPK alpha 2 mutants, or the use of a CDK4 inhibitor, increased FAO rates in MEFs and myotubes. In addition, Cdk4(-/-) mice have increased oxidative metabolism and exercise capacity. Inhibition of CDK4 mimicked these alterations in normal mice, but not when skeletal muscle was AMPK deficient. This novel mechanism explains how CDK4 promotes anabolism by blocking catabolic processes (FAO) that are activated by AMPK.