Locally released endothelin-1 has been recently identified as an important mediator of cardiac hypertrophy. It is still unclear, however, which primary stimulus specifically activates endothelin-dependent signaling pathways. We therefore examined in adult rats (n=51) the effects of a selective ET_A receptor antagonist in experimental models of cardiac hypertrophy, in which myocardial growth is predominantly initiated by a single primary stimulus. Rats were exposed to mechanical overload (ascending aortic stenosis), increased levels of circulating angiotensin II (angiotensin II infusion combined with hydralazine) or adrenergic stimulation (infusion of norepinephrine in a subpressor dose) for 7 days. All experimental treatments significantly increased left ventricular weight/body weight ratios compared with untreated rats, while systolic left ventricular peak pressure was increased only after ascending aortic stenosis. ET_A receptor blockade exclusively reduced norepinephrine-induced cardiac hypertrophy and ANP gene expression. Blood pressure levels and heart rates remained unaffected during ET_A receptor blockade in all experimental groups. These data indicate that in rat left ventricle the endothelin-dependent signaling pathway leading to early development of cardiac hypertrophy and fetal gene expression is primarily activated by norepinephrine.