Endothelin-1 (ET-1), a potent vasoconstrictor peptide synthesized by the vascular smooth muscle endothelium, has been previously shown to produce a sustained, salt-sensitive elevation in mean arterial pressure when chronically infused over a 7-day period into male Sprague-Dawley rats. In addition to other physiological actions, ET-1 has been shown to have potent effects on various renal functions, including renin production. Activation of the renin-angiotensin system, therefore, may contribute to the pressor response induced by ET-1. In this investigation, captopril ([2S]-l-[3-mercapto-2-methylpropionyl]-Lproline), a sulfhydryl-containing angiotensin I converting enzyme inhibitor, was chronically administered to endothelin-infused rats to elucidate the role of the renin-angiotensin system in this animal model of hypertension. Rats were catheterized, housed in metabolic cages, and maintained on a fixed 6.0 meq • day" 1 sodium intake throughout the experiment, with daily measurements taken of mean arterial pressure, heart rate, water intake, urine output, and urinary sodium and potassium excretions. Infusion of ET-1 alone at a rate of 5.0 pmol • kg" 1 • min" 1 for 7 days was associated with a significant and sustained increase in mean arterial pressure; concomitant chronic administration of captopril in another group of rats at a rate of 1.0 mg • kg" 1 • hr~' prevented the ET-1-induced hypertension. In an additional study, however, increases in plasma angiotensin II concentration were not observed in rats administered ET-1 alone at 5.0 pmol • kg" 1 • min~'. These results indicate that endothelin-induced hypertension may involve stimulation of the renin-angiotensin system but not an increase in circulating angiotensin II concentration. (Hypertension 1992;19:676-680) KEY WORDS • endothelin • captopril • renin-angiotensin system • chronic hypertension • rat studies S ince the isolation of the endothelial-produced, vasoactive peptide endothelin in 1988, extensive investigations into its biochemical and physiological nature have been performed. Presently, although the biochemical nature of this family of peptides has been extensively described in the available literature, 1 much of the physiology of the endothelins remains to be elucidated. Likewise, the relevance of circulating or endothelial-derived endothelin in cardiovascular physiology or pathophysiology has not been firmly established. Considerable evidence exists, however, that suggests a possible pathophysiological role for circulating endothelin.2 For example, recent reports implicate the endothelin-1 (ET-1) isoform in several diseases, including cerebral ischemia, 3 myocardial infarction, 4 congestive heart failure, 5 hypertension, 6 and hemangioendothelioma (endothelin-secreting tumor), 7 as these clinical situations were associated with significantly elevated plasma levels of endothelin. Alternatively, because this vasoactive peptide is produced ubiquitously throughout the vascular endothelium 8 and other cardiovascular structures, 9 there is also reason ...