To examine the rates of whole body protein synthesis and energy expenditure during the rapid growing period, premature infants of very low birth weight (VLBW) (less than 1500 g), appropriate for gestational age were kept under standard thermoneutrality conditions and received a formula diet providing 110 kcal/kg.d metabolisable energy (ME) and 3.3 g protein/kg.d. Their energy expenditure was measured by open circuit indirect calorimetry. Nitrogen turnover and whole body protein synthesis and catabolism were determined using repeated oral administration of 15N-glycine for 60-72 h followed by the analysis of 15N-enrichment in urinary urea. These VLBW infants grew at an average rate of 15 g/kg.d. About half of the ME intake (i.e. 50 kcal/kg.d) was invested in weight gain while the remainder (i.e. 60 kcal/kg.d) was oxidised. The energy equivalent of the weight gain (i.e. the amount of energy stored per g weight gain) and the N balance indicated that lean tissue made up approximately 2/3 of the weight gained and fat tissue the remaining 1/3. The plateau value for 15N enrichment reached on the third day of administration allowed us to calculate a rate of protein synthesis of 14 g/kg.d and protein breakdown of 12 g/kg.d in five VLBW fed a formula diet. The elevated energy expenditure of the very low birth weight infant seems to be related to its rapid rate of weight gain which is accompanied by a high rate of body protein synthesis. More than 20% of the total energy expenditure of the VLBW infants was accounted for by whole body protein synthesis.