Objectives To analyze the relative effects of antidiuretic hormone (ADH) and the extracellular fluid (ECF) volume on the dynamics of urinary excretion of urate.Design and Methods The fractional urate excretion (FE U A ) was surveyed in two patients with central diabetes insipidus (DI), and in one patient with syndrome of inappropriate ADH secretion (SIADH).Results The two cases with DI showed hyperuricemia (7.8 mg/dl, 8.2 mg/dl), and the SIADH case showed hypouricemia (2.4 mg/dl). At the end of the water-restriction test in DI patients, ADH (5U) was injected subcutaneously. FE U A fell from 3.5% to 2.4% in one patient, and from 3.3% to 2.3% in the other. In the case of SIADH, however, FE U A was relatively higher (13.5%). During the treatment of SIADH by oral salt replacement and water restriction, FE U A fell gradually to 6.3%.Conclusion1. The urinary excretion of urate was a dominant factor in determining the serum urate level.2. FE U A was mainly affected by the ECF volume.3. ADH played a role in modifying the regulation of urate excretion.