Background. Arginine vasopressin (AVP) is a key hormone in regulating blood pressure and body water balance, but despite modern procedures remains difficult to measure. Copeptin, the carboxyl-terminus portion of the AVP precursor, is much more stable and presents a reliable alternative to measuring the secretion of AVP. Copeptin has been studied to great extent in adults and is emerging as a useful diagnostic and prognostic biomarker for a variety of diseases. As of now there has been no research on copeptin in pediatric medicine. The aim of this study was to assess the normal basal values of plasma copeptin in infants in the first days of life and to investigate influencing factors at birth and during early postnatal adaptation.