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Using two different coimmunoprecipitation strategies as well as bioluminescence resonance energy transfer (BRET) techniques, we determined that the human oxytocin receptor forms dimeric and oligomeric complexes in vivo in intact living cells, and that these complexes exist at the cell surface level. Using a BRET-based assay, we found that oligomers can form between oxytocin receptors themselves (homo-oligomers) as well as, with a reduced affinity, between the oxytocin receptor and related members of the vasopressin receptor family (V1a and V2 receptors), but not with the more remotely related bradykinin receptor. The existence of oxytocin receptor oligomers at the level of the cell surface was demonstrated by a coimmunoprecipitation approach involving direct antibody exposure of intact living cells. Furthermore, this approach demonstrated that cell surface oxytocin receptor oligomerization is ligand independent. However, agonist addition led to an apparent rapid decrease in receptor oligomerization, as assessed by the coimmunoprecipitation approach, indicating that agonist exposure may modulate the oligomerization status. It remains to be determined to what extent oxytocin receptor oligomerization impacts on signal transduction.