Twenty exchange transfusions for hyperbilirubinaemia were performed via the umbilical vein or artery in 17 term and pre-term infants. Blood preserved with acid citrate and dextrose was used and the effect of the route of transfusion on plasma glucose, free fatty acid, insulin and growth hormone (GH) concentrations was measured during the transfusion and for 3 h afterwards. Infants transfused via the umbilical vein secreted more insulin than those transfused via the artery although both groups had similar rises in plasma glucose. After transfusion, glucose disappearance was faster and minimum plasma glucose levels were lower in infants transfused via the vein. This group had a smaller fall in plasma free fatty acids during transfusion, but otherwise plasma free fatty acid levels were similar in both groups. More GH was secreted by infants transfused via the artery resulting in higher plasma levels during and after transfusion.