The endothelin family of peptides are potent vasoconstrictors synthesized by endothelial cells in both constitutive and inducible pathways. The biological actions of endothelins are mediated by two GPCRs, ETA and ETB. ETA is the predominant receptor on smooth muscle cells and thus is the primary mediator of the vasoconstrictor activity of endothelins in vivo. Endothelin receptors are also found in the epithelium and central nervous system (Davenport, 2002). Cloned human ETA receptor-expressing ChemiScreen cells were constructed by stable transfection of Chem-1 cells with ETA. These stability-tested cells are ready for fluorescence-based assays for agonists, antagonists and modulators at the ETA receptor.