Angiotensin-II (AT-II) participates in the pathophysiology of cardiovascular diseases. Regulation of gap junctional intercellular communication may influence heart function and its response to cardiac injury. We examined the effects of Angiotensin-II on connexin43 (Cx43) and connexin40 (Cx40) in cultured neonatal rat ventricular cardiomyocytes and the role of mitogen-activated protein kinase signaling in the Angiotensin-II-induced responses. Cultured neonatal rat ventricular cardiomyocytes were incubated for 24 h with Angiotensin-II at concentrations ranging from 10 to 1000 nM. Cx43 expression and phosphorylation increased with increasing concentrations ofAngiotensin-II. Angiotensin-II-induced Cx43 induction could be completely inhibited by the AT1 receptor antagonist losartan. In contrast to Cx43, Cx40 expression did not change in AT-II-treated cultured neonatal rat ventricular cardiomyocytes . Thus, these two connexins were differentially regulated. Angiotensin-II increased the gap junctional conductance between the cardiomyocytes in culture as measured using a dual-cell voltage clamp. Mitogen-activated protein kinase inhibition revealed that both ERK and p38 signal pathways were involved in the regulation of Cx43 by Angiotensin-II. Thus, stimulation of the ERK and p38 signal pathways via AT1 receptors may partcipate in the regulation of cardiac gap junctions under (patho)physiological conditions.