Loss of function mutations of the type-2 vasopressin receptor (V2R) in kidney lead to nephrogenic diabetes insipidus (NDI). The function of misfolded mutant receptors can be rescued using pharmacochaperons, which promote plasma membrane localization of V2Rs. We identified with genetic analysis a previously described, but not characterized, mutation in a NDI patient. In order to determine personalized therapy for the individual, properties of the mutant receptor were characterized. Based on the findings we could propose a therapeutical strategy. The N321K mutation was identified with sequencing of the PCR product of genomic DNA isolated from peripheral blood. We constructed a highly sensitive Epac based BRET (bioluminescence resonance energy transfer) biosensor to perform real-time cAMP measurments. The β-arrestin binding of the receptors was examined with luciferase-tagged β-arrestin and mVenus-tagged V2Rs using BRET technique. The BRET measurements were performed on transiently transfected HEK293 cells using 96-well plates and Berthold Mithras LB 940 multilabel reader. Cell surface expression was determined with flow cytometry using anti-HA-Alexa488 antibodies. Localization examinations were implemented with fluorescent tagged receptors visualized with Zeiss LSM510 confocal laser-scanning microscope. The effect of agonists on V1R was tested on mouse arteries by wire myography. Previous studies have identified the pathogenic N321K mutation is in the 7th transmembrane helix of the V2R. Flow cytometry and confocal microscopy revealed that cell surface expression of the mutant receptor is similar to that of the wild type receptor. Determination of the ligand induced cAMP generation of the mutant receptor showed increased EC50 in arginine-vasopressin (AVP) stimulation and lack of signal in desmopressin (DDAVP) stimulation. BRET experiments revealed decreased β-arrestin binding of N321K-V2R. The mutant receptor also showed different sensitivity for V2R agonists. Valine-desmopressin (DVDAVP) relatively efficiently stimulated N321K-V2R, whereas even high doses of DVDAVP did not have effect on mouse arterioles, which are sensitive to AVP-induced vasoconstriction. N321K mutant V2R showed cell surface expression, the physiologically essential cAMP generation of the receptor can be activated with elevated dose of AVP, while with clinically used DDAVP was not efficient. Different internalization of the receptor may occur through altered β-arrestin binding. Function of the mutant receptor can be rescued with administration of V2R receptor agonist DVDAVP, which had no detectable side effects on V1R in the effective concentration. Our in vivo experiments suggest that DVDAVP can rescuse the function of the N321K-V2R in a NDI patient with no significant side effect on V1R. Supported by TÁMOP (4.2.1.B-09/1/KMR-2010-0001), OTKA 100883.