The pattern of arginine vasopressin (AVP) secretion in the immediate neonatal period is unclear. Plasma concentrations of AVP are reflected by its urinary excretion, thus providing a noninvasive method for studying the pattern of AVP release in the neonate. In these studies, we determined the pattern of urinary AVP excretion (microU/mg creatinine) during the first 2-4 days after birth in 78 neonates, 53 of whom had various prenatal and/or neonatal complications. In well term (n = 12) and preterm (n = 13) infants mean urinary AVP excretion decreased gradually during the first 24-36 h after birth. Although term and preterm infants with perinatal asphyxia had highest initial levels of urinary AVP (greater than 200 microU/mg creatinine) and a significant negative correlation with the 1-min Apgar score was obtained, their pattern of excretion was similar to respective controls. After delivery, elevated values for urinary AVP excretion were found among infants with neonatal courses complicated by intracranial hemorrhage, hypoxic encephalopathy, and pneumothorax. Urine osmolality did not correlate linearly with urinary AVP levels, but rather attained a maximum value of approximately 400 mosmol/kg at urinary AVP levels less than 200 microU/mg creatinine and then plateaued. It is concluded that the decrease in urinary AVP excretion observed soon after birth generally reflects diminution of the hypersecretion of AVP during parturition; neonates with evidence of intrapartum asphyxia initially have increased urinary AVP excretion; however, the pattern of excretion is similar to normal infants. During the neonatal period insults such as pneumothorax and intracranial hemorrhage may cause hypersecretion of this hormone.