The postnatal development of insulin secretion was studied in a group of premature infants (26-30 weeks gestation at birth) for periods up to 110 days after birth and in a small group of full-term infants (38-42 weeks gestation) for up to 47 days after birth. Circulating insulin levels were measured before, and at 30 min after the commencement of a glucose infusion given either parenterally or enterally depending on conceptual age and the mode of nutrition of the infant. The insulin response was assessed as the ratio of the increase in plasma insulin concentration to the increase in blood glucose concentration at 30 min. There was a small insulin response to glucose on day 1 in the premature infants and this increased slowly over the remainder of the study period regardless of the route of administration of glucose or of the mode of nutrition. The full-term infants were more responsive over the seven postnatal weeks in which they were studied. It is concluded that early premature infants can take up to eighteen weeks to develop an ability to respond fully to hyperglycaemia with insulin secretion even though full oral nutrition had been established over the last twelve weeks.