Endothelins (ETs) are 21 amino acid peptides which bind to ET A-and ET B-receptors to evoke diverse physiological responses. This report studies the internalization of ET A-receptor in Chinese hamster ovary (CHO) cells which were stably transfected with ET A-receptor cDNA. ET-1 binding induced ET A internalization in a time-dependent manner with 40% of ET A-receptors internalized at 37 C after 30 min. To localize internalized ET A-receptor, cells were immunostained using a polyclonal antibody against the extracellular loop between IV and V transmembrane segments of the ET A-receptor. To examine the fate of internalized ET-1, cells were treated with 10 nM biotinylated ET-1 coupled with Texas Redlabeled streptavidin. In the absence of ET-1, a majority of ET A was localized on the surface of cells. After ET-1 treatment for 60 min, internalized ET A-receptors were localized in a perinuclear structure. ET-1 remained bound to ET A-receptor after internalization for up to 60 min and then dissociated from the receptor. After dissociation, ET-1 possibly became degraded and ET A recycled back to the cell surface. Protein kinase inhibitors such as KT5926 and staurosporine partially inhibited ET A-receptor internalization. The results of this study may facilitate the understanding of pathways involved in ET-1-induced receptor internalization.