A potent peptidergic vasoconstrictor in vitro and in vivo termed endothelin has been isolated from the supernatant of cultured endothelial cells. The autoradiographic localization of 125J-endothelin has been studied after intravenous administration in rat tissues. Highest enrichment of radioactivity was found in kidney and lung. Activity was also detected, especially in vascular wall of the aorta and adrenal gland.