Male Brattleboro rats with hereditary diabetes insipidus (BDI) were lifetime-treated with the vasopressin V2 receptor agonist desamino-8D-arginine vasopressin (DDAVP), given daily in the drinking fluid. The DDAVP-treated adult male BDI rats drank 34 +/- 6 ml/24 h (mean +/- s.e.m.) and excreted urine volumes of 22 +/- 5 ml/24 h compared with their age-matched untreated controls of 142 +/- 12 and 115 +/- 7 ml/24 h respectively. There was no significant difference between the mean body weights of chronically DDAVP-treated BDI rats (198 +/- 9 g) and untreated animals (207 +/- 9 g). Morphometry of sections of kidney confirmed extensive hydronephrosis in the right kidneys of the control untreated Brattleboro rats only. This was quantified as the area of pelvis expressed as a percentage of total cross-sectional area of kidney (17 +/- 3 compared with 5 +/- 1% in the chronically DDAVP-treated rats; P less than 0.002). Medium-term treatment of adult BDI rats with DDAVP reduced daily fluid output towards normal rat values but hydronephrosis was still present. These observations indicate that the restoration of fluid balance in adult BDI rats by treatment from conception with DDAVP may be an important factor in preventing the development of hydronephrosis in these animals.