T he response of trained, conscious dogs to an acute water load was studied before adrenalectomy and under five conditions of hormonal replacement and sodium intake after adrenalectomy. Before adrenalectomy, with the dogs drinking isotonic saline, the minimal urinary osmolality (Uosm) was 47±7 (SEM) mOsm and free-water clearance (C_H2O) was 8.6±1 ml/min. These values were not different after adrenalectomy with or without deoxycorticosterone (DOCA) if the animals continued to drink saline and receive dexamethasone. Moreover, after adrenalectomy in the presence of saline drinking both dexamethasone and DOCA could be withdrawn for up to 4 days without impairment of diluting ability (Uosm, 54±7 mOsm and C_H2O, 7.3±1 ml/min). In contrast, when the dogs drank tap water (Na intake 30 mEq/day), water loading in the absence of dexamethasone and DOCA was associated with a significantly higher Uosm (127±28 mOsm) and lower C_H2O (2.7±0.3 ml/min). Replacing DOCA alone in the presence of this limited Na intake returned diluting ability to normal (Uosm 31±7 mOsm, C_H2O 7.7±0.5 ml/min). Glomerular filtration rate for each animal was the same under each condition except for a significant diminution which occurred when dexamethasone and DOCA were withdrawn while the animals were on a 30 mEq sodium intake. In contrast to previous conclusions, the present results indicate that in the absence of adrenal hormones normal renal diluting ability may occur, indicating both maximal suppression of vasopressin release and maximal distal tubular impermeability to water. In the present study the diluting defect observed after adrenalectomy related to negative sodium balance and could be overcome by either replacement with DOCA or a high intake of sodium alone.