Activation of adenosine monophosphate-activated protein kinase (AMPK) has been shown to inhibit cardiac hypertrophy through peroxisome proliferators-activated receptor-α (PPARα) signaling pathway, but the detailed mechanism remains unclear. A rat model of cardiac hypertrophy created by transaortic constriction (TAC) was used to investigate the mechanism involved in regulation of PPARα activity by AMPK. It was observed that treatment with AICAR (5-aminoimidazole 1 carboxamide ribonucleoside), an AMPK activator, significantly inhibited cardiac hypertrophy in vivo and in vitro. Phosphorylated extracellular signal regulated protein kinase (phospho-ERK1/2) and phospho-p38 mitogen-activated protein kinase (MAPK) protein levels were significantly up-regulated, while PPARα protein level was down-regulated in TAC rats. AICAR treatment reversed the changes of PPARα and phospho-ERK1/2, but increased phospho-p38 MAPK protein level in TAC rats. Similar changes of PPARα and phospho-ERK1/2 protein levels were observed in the hypertrophied cardiomyocytes induced by phenylephrine treatment. Epidermal growth factor (EGF, ERK1/2 activator), but not SB203580 (p38 inhibitor) blocked the up-regulation of PPARα protein level induced by AICAR. Luciferase assay showed that AICAR increased PPARα transcriptional activity which was abrogated by EGF, but not by SB203580. These results demonstrate that AMPK activation enhances the activity of PPARα to inhibit cardiac hypertrophy through ERK1/2, but not p38 MAPK, signaling pathway.