BACKGROUND: Urodilatin is a 32-amino-acid (AA) peptide formed in the kidney. METHODS: High-performance gel permeation chromatography and high-pressure liquid chromatography evaluation of plasma followed by sensitive urodilatin and atrial natriuretic peptide (ANP) assays revealed that urodilatin does circulate distinctly from ANP. RESULTS: Urodilatin circulates at very low levels (i.e 9-12 pg/ml). Infusion of ANP increased the circulating concentration of urodilatin 135-fold (p < 0.001), suggesting that some of the effects of ANP may be mediated by urodilatin while long-acting natriuretic peptide, vessel dilator, and kaliuretic peptide did not affect urodilatin in healthy humans (n = 30). Only ANP decreased the renal clearance of urodilatin (60-75%, p < 0.01). Urodilatin was metabolized into peptides smaller than 5 AAs as well as excreted intact into urine. CONCLUSION: Urodilatin circulates and is increased by ANP in humans.