We have investigated the function of the C-terminal and the third intracellular domains of the ETA receptor by expressing truncated and mutated ETA receptors in COS-7 and CHO cells. All the C-terminal truncated ETA receptors were produced at a similar expression level and were detected in the cell membrane using indirect immunostaining. The sizes of the truncated ETA receptors were decreased in proportion to the molecular mass of the truncated amino acid sequence. When the ligand binding activities were determined for various truncated ETA receptors, it was found that more than eight amino acid residues at the proximal cytoplasmic tail of the ETA receptor were required for ET-1 binding. In addition, the deletion of 16 C-terminal amino acid residues from the third intracellular loop severely decreased the ligand binding activity. It seems that deletion of these cytoplasmic domains of the ETA receptor influences the three-dimensional structure of the ligand binding site located in the extracellular domains. The ETA receptor required more than 13 amino acid residues in the proximity of C-terminal cytoplasmic tail and 10 amino acid residues in the C-terminal region of the third intracellular loop to induce the ET-1 dependent increase in intracellular calcium concentration. Both regions are possibly coupled with G-protein to transmit the ET-1 signal.