Mitogen-activated protein kinase kinase (MKK)4 is a critical member of the mitogen-activated protein kinase family. It is able to activate the c-Jun NH₂-terminal protein kinase (JNK) and p38 mitogen-activated protein kinase in response to environmental stresses. JNK and p38 are strongly implicated in pathological cardiac hypertrophy and heart failure; however, the regulatory mechanism whereby the upstream kinase MKK4 activates these signaling cascades in the heart is unknown. To elucidate the biological function of MKK4, we generated mice with a cardiac myocyte-specific deletion of mkk4 (MKK4^cko mice). In response to pressure overload or chronic -adrenergic stimulation, downregulated NFAT (nuclear factor of activated T-cell) transcriptional activity associated with exacerbated cardiac hypertrophy and the appearance of apoptotic cardiomyocytes were observed in MKK4^cko mice. However, when subjected to swimming exercise, MKK4^cko mice displayed a similar level of physiological cardiac hypertrophy compared to controls (MKK4^f/f). In addition, we also discovered that MKK4 expression was significantly reduced in heart failure patients. In conclusion, this study demonstrates for the first time that MKK4 is a key mediator which prevents the transition from an adaptive response to maladaptive cardiac hypertrophy likely involving the regulation of the NFAT signaling pathway.