Little is known of the ability of hypoglycaemic infants who are small for gestational age (SGA) to mount the coordinated hormonal and metabolic counterregulatory response that is seen in healthy older subjects during glycopenia. This response was studied in 22 SGA infants (birth weight < 10th centile) by measuring the blood concentrations of glucose, intermediary metabolites, and glucoregulatory hormones. Plasma non-esterified fatty acid and blood ketone body concentrations were low, even when blood glucose concentrations were low. Plasma insulin and glucagon varied widely (< 1.0-53.1 mU/l and 16.6-87.1 pmol/l, respectively). Concentrations of noradrenaline and glucagon were raised, but cortisol and adrenaline were lower than those found in hypoglycaemic adults. There was no relationship between the concentration of any hormone and blood glucose concentration. We postulate that hypoglycaemia and the failure to mobilise alternative fuels in some SGA infants is secondary both to a poorly coordinated counterregulatory hormone response and to a peripheral insensitivity to the actions of the hormones. Those infants, who fail to mount a counterregulatory response, should be identified by accurate and reliable blood glucose monitoring, and an adequate exogenous supply of energy, either enteral or parenteral, should be ensured.