Energetics and mechanics of sucking in preterm and term neonates were determined by simultaneous records of intraoral pressure, flow, volume, and work of individual sucks. Nine term infants (mean postconceptional age: 38.6 +/- 0.7 SD weeks; mean postnatal age: 18.4 +/- 6.1 SD days) and nine preterm infants (mean postconceptional age: 35.2 +/- 0.7 SD weeks; mean postnatal age: 21.9 +/- 5.4 SD days) were studied under identical feeding conditions. Preterm infants generated significantly lower peak pressure (mean values of 48.5 cm H2O compared with 65.5 cm H2O in term infants; P less than 0.01), and the volume ingested per such was generally less than or equal to 0.5 mL. Term infants demonstrated a higher frequency of sucking, a well-defined suck-pause pattern, and a higher minute consumption of formula. Energy and caloric expenditure estimations revealed significantly lower work performed by preterm infants for isovolumic feeds (1190 g/cm/dL in preterm infants compared with 2030 g.cm/dL formula ingested in term infants; P less than 0.01). Furthermore, work performed by term infants was disproportionately higher for volumes greater than or equal to 0.5 mL ingested. This study indicates that preterm infants expend less energy than term infants to suck the same volume of feed and also describes an objective technique to evaluate nutritive sucking during growth and development.