The aim of our study was to investigate whether urodilation (URO, INN: ularitide) is present in rat urine and if URO excretion in the rat is influenced by dietary sodium intake. Therefore, three groups of Wistar rats were placed in metabolic cages where they received different sodium diets for 9 days (0.05%, 0.4%, and 8.0% NaCl, respectively). Food and water intake were determined by weight. At days -4, 2, 5, and 8 blood pressure was measured non invasively using the tail cuff method. After nine days rats were anesthetized and blood was drawn for serum electrolyte, plasma A-type natriuretic peptide (CDD/ANP-99-126), and plasma aldosterone concentration measurements. Using a highly specific antibody against URO combined with high performance liquid chromatography and gel chromatography, we were able to show that a URO-like substance of approx. 3.5 kD that is distinct from CDD/ANP-99-126, brain natriuretic peptide, and C-type natriuretic peptide, is present in rat urine. Sodium chloride loaded rats showed significantly increased urinary excretion rates of URO (p < 0.001), chloride (p < 0.001), sodium (p < 0.001), and the fractional excretion of sodium (p < 0.001). In the plasma, sodium (p < 0.01) and chloride (p < 0.001) increased, while potassium, hematocrit, osmolality, plasma CDD/ANP-99-126, as well as glomerular filtration rate (GFR), and systolic blood pressure did not change. Since CDD/ANP-99-126 is believed to be a natriuretic peptide, it is suggested that CDD/ANP-99-126 might participate in the natriuresis due to high dietary sodium intake. In sodium-loaded rats, however, plasma CDD/ANP-99-126 remains unchanged, while URO excretion increases with sodium excretion, independent of GFR and blood pressure. We conclude that URO secretion is stimulated by dietary salt loading and might be involved in the regulation of water and electrolyte metabolism in the rat.