Using a cre-loxP-mediated gene-switch approach, we achieved targeted JNK activation in adult hearts. A transgenic model is established carrying a floxed gene-switch construct that directs GFP marker gene expression in the absence of DNA recombination between two loxP sites. A tamoxifen-inducible Cre recombinase was introduced in the transgenic heart by breeding with previously established Mer-Cre-Mer transgenic mice. Upon tamoxifen administration in double transgenic adult animals, cre-loxP-mediated DNA recombination efficiently switches "off" the loxP-flanked GFP expression unit in cardiomyocytes and switches "on" the expression of the target gene, MKK7D, a constitutively activated upstream activator of c-Jun N-terminal kinases (JNK). Expression of MKK7D in adult hearts resulted in significant activation of JNK activities and causes progressive cardiomyopathy in transgenic animals. This unique animal model of cardiac-specific and temporally regulated JNK activation will provide a powerful tool to investigate the functional role of the JNK pathway in the development of heart failure. Our data also demonstrated that the inducible gene-switch approach reported here may also be applicable in other studies to achieve efficient, tissue-specific, and temporally regulated genetic manipulation in intact animals.