A renal natriuretic peptide and the ‘renal urodilatin system’ were identified after the observation that immunoassayable ANP in urine may not be identical to the circulating cardiac hormone ANP, which is a peptide of 28 amino acids. Urodilatin (INN: Ularitide) is a natriuretic peptide isolated from human urine and belongs to the family of A-type natriuretic peptides. Urodilatin is differentially processed to a peptide of 32 amino acids from the same precursor as ANP. It is synthesized in kidney tubular cells and secreted luminally. After secretion from epithelial cells of the distal and/or connecting tubules, Urodilatin interacts downstream at distal segments of the nephron with luminally located receptors whereby it regulates Na⁺ and water reabsorption. Thus, the physiological function of the renal Urodilatin system can be described as a paracrine intrarenal regulator for Na⁺ and water homeostasis, considering Urodilatin as a real diuretic-natriuretic regulatory peptide. However, the regulation upon which the Urodilatin secretion depends is still not clear. Since Urodilatin has been discovered, a great number of pharmacological and clinical investigations have been carried out using Urodilatin as a drug for several indications. So far, clinical phase I and II studies for acute renal failure, congestive heart failure, and bronchial asthma have been performed.