Small substrate stores and immature enzyme systems make preterm infants prone to develop hypoglycemia. Hyperglycemia may also occur, particularly when these infants are given i.v. glucose. To evaluate the capacity for regulation of glucose production in response to glucose infusion, 10 newborn infants, born after 25-30 wk of gestation, were studied. Their glucose production rates (GPR) were calculated and the concentrations of glucose, insulin, and glucagon in plasma were measured during infusion of glucose at a rate of first 1.7 +/- 0.2 and then 6.5 +/- 0.3 mg.kg-1.min-1 (9.4 +/- 1.1 and 36.1 +/- 1.7 mumol.kg-1.min-1) (mean +/- SD). GPR was determined by use of D-6,6-2H2-glucose. When the rate of infusion of glucose was increased, GPR decreased from 4.3 +/- 1.3 to 1.4 +/- 1.1 mg.kg-1.min-1 (23.9 +/- 7.2 to 7.8 +/- 6.1 mumol.kg-1.min-1) (mean +/- SD) (p = 0.00006). In addition, the plasma insulin concentration increased from 6 +/- 2 to 11 +/- 4 microU.mL-1 (p = 0.006) and the plasma glucose concentration from 3.6 +/- 1.1 to 6.1 +/- 1.3 mM (mean +/- SD) (p = 0.0002), whereas the glucagon concentration remained unchanged. Only the insulin concentration in plasma was significantly related to GPR. The results show that very immature newborn infants have an incomplete and varying capacity to respond to glucose infusion with suppression of glucose production. Insulin seems to be more important than plasma glucose in the regulation of glucose homeostasis in these infants.