BACKGROUND Urodilatin (URO) is a member of the natriuretic family, cleaved by the kidney, which acts as a paracrine hormone in the regulation of natriuresis and diuresis. In newborn infants the excretion of urodilatin and its biological effects have not been explored. METHODS We measured urinary URO excretion, by direct RIA (radioimmunoassay), as well as its correlation to neonatal body weight loss, and sodium homeostasis in 30 full-term newborn infants on the 4th day of life. RESULTS The URO excretion, estimated as URO:creatinine ratio, was significantly correlated to sodium excretion. CONCLUSION These data show that in full-term newborn infants the mechanisms that control synthesis, excretion and signal transduction of URO are developed and that URO contributes to natriuresis regulation.