Renin and renin-substrate concentrations were measured in the paired umbilical artery and vein blood of 20 babies born by elective caesarean section, and 36 babies born vaginally. Blood pH, carbon dioxide tension and oxygen tension were measured in all samples. Babies born vaginally were significantly more acidaemic than those delivered by caesarean section. The cord artery blood also had a significantly greater carbon dioxide tension at vaginal delivery. A decrease in pH of either cord artery or vein blood, irrespective of mode of delivery, was associated with an increase in renin concentration. Renin-substrate concentration tended to be lower in the cord vein blood of acidaemic babies born vaginally, such that the ratio renin: renin-substrate concentration was negatively correlated with pH. At caesarean section the renin concentration of cord vein blood was positively correlated with carbon dioxide tension and increased dramatically at oxygen tensions less than 3kPa. This was not observed in babies born vaginally, but the cord artery substrate concentration in these infants was positively correlated with carbon dioxide tension. Thus changes in the renin-angiotensin system at birth may partly be affected by blood gas tension.