OBJECTIVE: To develop radioimmunoassay for aquaporin-2 (AQP-2). METHODS: Anti-AQP-2 antiserum has been raised in New Zealand white rabbits immunized with a conjugate of synthetic AQP-2 peptide (257-271) with bovine serum albumin. Radioiodination of synthetic peptide (tyrosine-AQP2 (257-271) was performed by chloramine T method, followed by purification of radioiodinated material on Sephadex G-25 column. RESULTS: The obtained antibody did not crossreact with vasopressin, pituitary hormones, hypothalamic hormones and neuropeptides. The assay was performed with a double antibody system. The values are expressed as an equivalent of synthetic AQP-2 peptide (257-271). The dilution curve of high AQP-2 urine in radioimmunoassay system was parallel to the standard curve. The recovery percentage of AQP-2 added to urine was about 100 % in this assay system. Intra-assay and inter-assay variation was 4.5 % and 7.2 %, respectively. Mean urinary excretion of AQP-2 was 1.16 ng equivalent of AQP-2 (257-271)/mg creatine and was lower in patients with diabetes insipidus. CONCLUSION: These data suggest that his assay system is a suitable to measure AQP-2 in urine.