Endothelin-1 (ET-1) induces contraction of vascular smooth muscle through binding to endothelin type A receptor (ET AR). COS-7 cells stably expressing high levels of the ET AR were established (designated COS-7(ET AR)). The COS-7(ET AR) cell bound 125IET-1 with a K d of 932 ± 161 pM and a B max of 74 ± 13 fmol/2 × 10 5 cells. 125IET-1 binding was inhibited by ET-1 and the ET AR antagonist BQ-610, but not by the endothelin type B receptor (ET BR) antagonist BQ-788. In clones expressing two ET AR mutants containing D46N or R53Q substitutions in the first extracellular domain of the receptor, 125IET-1 binding activity was dramatically reduced. This suggests that these single amino acid substitutions alter the three-dimensional structure of the ligand-binding domain of the ET AR. Using COS-7(ET AR) cell, we showed that Ca 2+ or Mg 2+ was essential for ET-1 binding to the ET AR and that ET-1 treatment induced postreceptor signaling, that is, intracellular accumulation of cyclic AMP (cAMP) and Ca 2+ mobilization. The COS-7(ET AR) established in this study will be a useful tool for screening ET-1 antagonists for treating hypertension.