Endothelin-1 (ET-1) has been implicated in hypertension, heart failure, atherosclerosis, and pulmonary hypertension. In all these conditions, plasma immunoreactive ET-1 levels are elevated, and tissue ET-1 expression is increased. Clinical trials have demonstrated potentially important benefits of ET antagonism among patients with essential hypertension, pulmonary hypertension, and heart failure. It is unknown whether ET antagonism affects the production of ET-1 in stroke-prone spontaneously hypertensive rat (SHRSP) heart at the typical hypertensive stage. The objective of this study was to investigate the effects of ET blockade on the expression levels of plasma and cardiac ET-1 in SHRSPs. SHRSPs were treated for 3 months with SB209670 (ET(A)/ET(B) dual receptor antagonist) or with saline (vehicle) commencing at the prehypertensive stage (age 6 weeks). Plasma and left ventricular ET-1 peptide levels were measured using enzyme-linked immunoabsorbent assay. Compared with age-matched control Wistar-Kyoto rats, peptide levels of ET-1 were significantly upregulated in vehicle-treated SHRSP heart; this upregulation was reversed by long-term ET antagonism. Plasma ET-1 levels were also significantly increased in vehicle-treated SHRSPs and were normalized by ET antagonism. mRNA expression of preproET-1, which is the source of ET-1 peptide production, was significantly increased in vehicle-treated SHRSP heart and was normalized by ET antagonism. Marked cardiac hypertrophy and fibrosis at the histologic level in SHRSPs were ameliorated by ET antagonism, and left ventricular hypertrophy as seen on echocardiography in SHRSPs was suppressed by ET blockade. After ET antagonism, systolic blood pressures were reduced in SHRSPs; diastolic blood pressures were unchanged. The reversal effect of the upregulated ET system in SHRSP heart by ET antagonism might be independent of blood pressure change. By suppressing the upregulated ET system, ET antagonism might be beneficial in arresting cardiac remodeling.