The recently discovered 32 amino-acid natriuretic peptide urodilatin was chemically synthesized and subjected to two-dimensional proton nuclear magnetic resonance (NMR) spectroscopy studies in aqueous solution in order to determine the structural state of urodilatin. In contrast to earlier studies on very closely related peptides, such as cardiodilatin (CDD/ANP-99-126) and brain natriuretic peptide (BNP), spectra of urodilatin were extremely well resolved even in millimolar concentration in H2O so that the complete sequence specific resonance assignments could be achieved. No long range NOEs could be detected, except between residues close to the single cystine bond. This leads to the conclusion that urodilatin in aqueous solution is a random coil peptide with the exception of the region around the cystine bond.