Administration of urea to patients with the syndrome of inappropriate antidiuresis (SIAD) is thought to ameliorate hyponatremia by both producing an osmotic diuresis and diminishing ongoing natriuresis. The present study evaluated these effects in a rat model of SIAD utilizing dilutional hyponatremia induced by continuous infusion of 1-deamino-[8-D-arginine] vasopressin. Following 48 hours of sustained hyponatremia, separate groups of rats were then refed with either: (1) 5% dextrose alone, (2) a 20% protein chow, (3) an isocaloric protein deficient (0%) chow, or (4) the isocaloric protein-deficient chow supplemented with oral urea. Our results demonstrate that rats refed a 20% protein diet significantly improved their plasma [Na+] as compared to rats refed protein deficient diets, and this improvement was accompanied by decreases in natriuresis despite an increased glomerular filtration rate and an unchanged negative free water clearance. Identical effects were observed in rats refed a protein deficient diet but supplemented with oral urea, suggesting that urea generation from catabolism of dietary protein is responsible for the effect of protein refeeding to decrease urinary sodium excretion. Both the protein and urea refed rats had significantly higher inner medullary urea contents and concentrations compared to rats refed protein-deficient diets and also to rats studied immediately before protein refeeding, supporting the hypothesis that urea and dietary protein decrease natriuresis in patients with SIAD in association with increased inner medullary urea concentrations.