Many lines of evidence have suggested that angiotensin II (Ang II) plays an important role in cardiac hypertrophy. Ang II not only increases protein synthesis but also induces the reprogramming of gene expression in cultured cardiac myocytes. In the present study, to elucidate the mechanism by which Ang II regulates gene expression in cardiac myocytes, we examined whether Ang II activates c-Jun NH₂-terminal kinase (JNK), which is a member of the mitogen-activated protein kinase family and activates the transcription factor, activator protein-1 (AP-1). The activity of JNK increased 5 minutes after the addition of Ang II, peaked at 20 minutes, and gradually decreased thereafter. Examination of the Ang II dose-response relation revealed detectable JNK activation at 10⁻⁹ mol/L and maximal activation at 10⁻⁶ mol/L. Ang II activated JNK through the AT₁ receptor, and the activation was attenuated by the downregulation of protein kinase C or the chelation of intracellular Ca²⁺. Although the addition of either Ca²⁺ ionophore or phorbol ester resulted in little or no activation of JNK, simultaneous addition of both Ca²⁺ ionophore and phorbol ester markedly activated JNK. Slight expressions of the c-jun gene were observed in unstimulated cardiac myocytes, and Ang II increased expressions of the c-jun gene as well as the c-fos gene. Ang II increased transcription of the endothelin-1 gene through the AP-1 binding site. In conclusion, Ang II may activate JNK in cultured cardiac myocytes through an increase in intracellular Ca²⁺ and activation of protein kinase C, and the activated JNK may regulate gene expression by activating AP-1 during Ang II–induced cardiac hypertrophy.