Agonist induces substantial changes in a cytoplasmic tail of G-protein coupled receptors. However, structural consequences of such changes have not been detected in V1a receptors, which was stimulated by full agonist. Here, we explored receptor-receptor interaction by using NanoBit split luciferase system. One of two parts of split luciferase were connected to the carboxy-terminal tail of the V1a receptor. These modifications of V1a tails did not alter vasopressin-mediated intracellular calcium signals. When V1a receptors were connected with small or large fragment of nanoluciferase and were co-expressed, strong luminescent signal was detected, indicating that V1a receptor tails formed full nanolucuferase molecule. We found that signal intensities of luciferase light gradually increased during measurement period of five minutes in human embryonic kidney cells after substrate additions. Vasopressin significantly reduced the luciferase signal intensity during the measurement period. Our data indicate that split nanoluciferase system is useful to detect small conformational changes in the cytoplasmic tails of the agonist-stimulated V1a receptor homodimers.