Abstract Angiotensin II (Ang II)-stimulated expression of endothelin-1 (ET-1) mRNA is blocked by ET Aantagonists in vitro.We studied effects of Ang II (200 ng/kg/min) and ET Aantagonist LU135252 (50 mg/kg/d) in WKY rats in vivoinvestigating vascular and renal ET-1 protein expression, functional endothelin converting enzyme (ECE) activity, and clearance of 125IET-1. Infusion of Ang II for two weeks increased ET-1 protein content in aorta (4.7-fold) and femoral artery (1.6-fold) with and without endothelium and in kidneys (3-fold, p<0.05) and enhanced functional ECE activity (p<0.05). The Ang II-induced increase in tissue ET-1 content and functional ECE activity was completely prevented by LU135252 (p<0.05). Chronic treatment of control animals with LU135252 lowered basal vascular but not renal ET-1 content (p<0.05 vs. control). Clearance of 125IET-1 was unaffected by the treatments. It is concluded that Ang II increases ET-1 protein and functional ECE activity in vascular smooth muscle and kidney through ET A-receptors in vivo.