Vasopressin concentrations were measured in the cord blood of thirty infants, thirteen delivered vaginally and seventeen by Caesarean section. There was no correlation with maternal values but the concentration following vaginal delivery (13.5 microunits/ml +/- 7.9 SD) was significantly higher than that following Caesarean section (4.2 microunits/ml +/- 6.6 SD). Gestational age did not affect the concentrations, which fell during the first week of life and then rose gradually in term and pre-term infants. High levels were found in seven sick babies. There was no correlation in the first 3 weeks of life between plasma vasopressin and plasma or urine osmolality in well and sick babies.