Accelerated glycolysis in hypertrophied hearts may be a compensatory response to reduced energy production from long chain fatty acid oxidation with 5'-AMP activated protein kinase (AMPK) functioning as a cellular signal. Therefore, we tested the hypothesis that enhanced fatty acid oxidation improves energy status and normalizes AMPK activity and glycolysis in hypertrophied hearts. Glycolysis, fatty acid oxidation, AMPK activity, and energy status were measured in isolated working hypertrophied and control hearts from aortic-constricted and sham-operated male Sprague-Dawley rats. Hearts from halothane (3-4%) anesthetized rats were perfused with Krebs-Henseleit solution containing either palmitate, a long chain fatty acid, or palmitate plus octanoate, a medium-chain fatty acid whose oxidation is not impaired in hypertrophied hearts. Compared to control, fatty acid oxidation was lower in hypertrophied hearts perfused with palmitate, while it increased to similar values in both groups with octanoate plus palmitate. Glycolysis was accelerated in palmitate-perfused hypertrophied hearts and was normalized in hypertrophied hearts by addition of octanoate. AMPK activity was increased 3 to 6-fold with palmitate alone and was reduced to control values by octanoate plus palmitate. Myocardial energy status improved with addition of octanoate, but did not differ between groups. Our findings, particularly the correspondence between glycolysis and AMPK activity, provide support for the view that activation of AMPK is responsible, in part, for the acceleration of glycolysis in cardiac hypertrophy. Additionally, they indicate myocardial AMPK is activated by energy state-independent mechanisms in response to pressure overload, demonstrating AMPK is more than a sensor of the heart's energy status.