Glucose is the predominant cerebral energy source under physiological conditions, although other substrates may support cerebral metabolism. The present study was undertaken to determine if lactate is present in the immature human brain, and if so, whether or not concentrations of lactate differ between small-for-gestational-age and appropriate-for-gestational-age infants. Thirty stable, healthy infants with normal brains were investigated. As the only nutrient, all received milk enterally prior to the investigation, which was carried out without sedation. Mean gestational age was 35 completed weeks (range 28-41 weeks) and mean birth weight was 2170 g (range 855-4100 g). Proton nuclear magnetic resonance spectra from the striatal region were obtained while the infants were sleeping quietly. Lactate was present in all 10 preterm small-for-gestational-age and 10 of 13 preterm appropriate-for-gestational-age infants, and the concentration was inversely related to postmenstrual age (p < 0.002). In addition, lactate increased with the degree of growth retardation (p < 0.01). At present the significance of lactate is unclear. Lactate may be produced locally or in peripheral tissues, and may support brain metabolism.