We examined the process of receptor-mediated endocytosis in A-9 lung fibroblast and chinese hamster ovary (CHO) cells transfected with the recently cloned vasopressin V1a receptor (51). We used a fluorescent labeled vasopressin analog (rhodamine-mercaptopropionic acid lysine vasopressin) and radiolabeled vasopressin to examine this process in the two transfected cell lines. Both A-9 and CHO cells internalize vasopressin in a manner consistent with receptor-mediated endocytosis. A-9 cells internalize vasopressin more rapidly than CHO cells. The process is inhibited by vasopressin and by specific vasopressin V1 receptor antagonists but not by specific V2 receptor antagonists. Hypertonic sucrose inhibits endocytosis in both cell types suggesting a role of clathrin coated pits in the endocytosis of receptor in these cells. These cells are excellent models in which to examine the effect of receptor mutations on vasopressin receptor-mediated endocytosis.