Oxytocin (OXT) and arginine vasopressin (AVP) are structurally similar neuropeptide hormones that function as neurotransmitters in the brain， and have opposite key roles in social behaviors. These peptides bind to their G protein-coupled receptors (OXTR and AVPRs)， inducing calcium ion-dependent signaling pathways and endocytosis of these receptors. Because selective agonists and antagonists for these receptors have been developed as therapeutic and diagnostic agents for diseases such as psychiatric disorders， facile methods are in demand for the evaluation of selectivity between these receptors. In this study， we developed a quantitative assay for OXT- and AVP-induced endocytosis of their receptors. The mutated Oplophorus luciferase， nanoKAZ， was fused to OXTR and AVPRs to enable rapid quantification of agonist-induced endocytosis by bioluminescence reduction. Agonist stimulation significantly decreases bioluminescence of nanoKAZ-fused receptors in living cells. Using this system， we evaluated clinically used OXTR antagonist atosiban and a reported pyrazinyltriazole derivative， hereby designated as PF13. Atosiban acted as an antagonist of AVPR1a， as well as an agonist for AVPR1b， whereas PF13 antagonized OXTR more selectively than atosiban， as reported previously. This paper shows a strategy for quantification of agonist-induced endocytosis of OXTR and AVPRs， and confirms its potent utility in the evaluation of agonists and antagonists.