There is little information on the metabolic and endocrine responses to milk feeding in the neonatal period particularly in relation to the model of nutrition and composition of the milk. Plasma concentrations of insulin, glucagon and gastric inhibitory polypeptide (GIP) together with blood levels of glucose, ketone bodies, pyruvate, lactate and glycerol were measured pre- and post-prandially in 79 health six-day-old term infants who had been either breast fed or fed on a modified cow's milk formula (Cow and Gate Premium) from birth. Formula fed infants had a greater insulin and GIP response to feeding and their basal and postprandial blood ketones were considerably lower than in breast fed infants. In addition a significantly greater post feed rise in both lactate and pyruvate concentrations was observed with formula feeding. These results may have significant implications regarding infant feeding and postnatal metabolism.