Background: Oxytocin is a nonapeptide hormone secreted from posterior pituitary gland and has a very similar structure to vasopressin. The aquaporin-2 (AQP2) water channel is predominantly expressed in the kidney and plays a key role in regulation of water permeability of mammalian collecting duct, exerted by both short-term and long-term vasopressin action. We speculated that oxytocin may be involved in some part of vasopressin-independent urinary concentrating mechanism by regulating AQP2 trafficking in the kidney. Methods: This study was undertaken to investigate whether and how the acute stimulation of oxytocin induces changes in AQP2 localization in the kidney. Immunohistochemistry and semiquantitative immunoblotting of AQP2 were carried out from Sprague-Dawley rat kidneys after a single intraperitoneal injection of oxytocin with or without pretreatment of a vasopressin-2 receptor (V2R) antagonist. Results: Urinary cAMP excretion was increased by oxytocin administration. Immuno- histochemistry of inner medullary collecting duct (IMCD) revealed that AQP2 was shifted from diffuse cytoplasmic localization in controls to the apical and basolateral membrane domains in oxytocin-treated rats. This pattern of AQP2 redistribution was noted in connecting tubule, cortical collecting duct and outer medullary collecting duct as in IMCD, although the tendency to basolateral localization was somewhat less. Semiquantitative immunoblotting of membrane fractions of whole kidney homogenates was also used t o assess redistribution of AQP2. The band density ratio of theplasma membrane-rich fraction over cyoplasmic vesicle- rich fraction was higher in oxytocin- treated rat s than in controls (3.64 ±0.60 vs. 1.09 ±0.14, p＜0.05＞. Regarding the receptor pathway of oxytocin action in the kidney, we found that pretreatment with a V2 R ant antagonist (OPC- 31260) blocked redistribution of AQP2 which was induced by oxytocin. Conclusion: In conclusion, oxytocin induces a V2 R- mediated redistribution of AQP2- containing cytoplasmic vesicles t o both apical and basolateral plasma membrane domains in r at kidney. Oxytocin may be one of the facts or that accounts for vasopressin - independent AQP2 t argeting in the kidney.(Korean J Med 62:268-277, 2002)