Nineteen exchange transfusions were performed via the umbilical artery using blood preserved with acid-citrate and dextrose in 8 infants of 34-40 weeks gestation (larger infants) and 9 very small infants of 26-33 weeks gestational age. The plasma glucose rise which was similar in both groups stimulated insulin secretion from the larger infants but not the very small infants. No significant differences occurred between the groups in the fall in mean free fatty acid levels or increase in growth hormone secretion. Following transfusion there was a sharp rise in mean plasma insulin concentration in the larger infants and a smaller rise in the very small infants. A highly significant positive correlation was found between the maximum posttransfusion plasma insulin and the birth weight of the infants. Plasma glucose levels of less than 30 mg/100 ml occurred in 2 larger and 5 very small infants during the first 3 hours after transfusion. One infant of birth weight 0.98 kg received four transfusions; in 2 where he received ACD blood via the umbilical artery or vein, insulin secretion was not stimulated but in the other 2 in which glucagon or arginine was added to the ACD donor blood, insulin secretion was stimulated. Feeding practice should take account of the fact that although very small infants secret less insulin than larger infants during exchange transfusion they are more likely to become hypoglycaemic in the immediate posttransfusion period.