Endothelial cells were harvested by the collagenase perfusion of isolated mesenteric arteries of rats and cultured. An endothelin peptide was detected in the supernatant of these cells by an antibody which recognizes ET-1 but not "rat" endothelin (ET-3). Culture media was extracted using a C-8 solid phase column and subjected to reverse phase HPLC using a system that separates all known endothelins and immunoreactive endothelins measured using another antibody which recognizes all endothelins. The main immunoreactive peak co-eluted with ET-1. We could not detect any ET-2, ET-3 or Vasoactive Intestinal Contractor. A smaller immunoreactive peak of unknown structure that eluted earlier than ET-1 was also detected. In conclusion, rat endothelial cells secrete a peptide of similar chromatographic and immunoreactive properties as ET-1.