SummaryLongitudinal studies on total and resting metabolic rate and total heat loss were made in 14 LBW infants, age 1-58 d. Metabolic rate was calculated from indirect calorimetry, heat loss was measured by direct calorimetry. Total metabolic rate and total heat loss were lowest during the first week of life, 178.9 ± 18.0 and 171.8 ± 15.5 kJ·kg-1·24 h-1, respectively. During the age period of 8-58 d both total metabolic rate and heat loss showed a very slight increase with age: mean total metabolic rate was 278.8 ± 2.6 and mean total heat loss 257.0 ± 3.4 kJ·kg-1·24 h-1. Resting metabolic rate was 171.0 ± 25.2 kJ· kg-124 h-1during the first week of life and 248.0 ± 2.5 during d 8-58.The energy cost of components of new tissue was calculated from the energy balance equation. Ecomponentsduring the second week of life was 25.5 ± 4.9 kJ/g weight gain and 11.9 ± 0.4 kJ/ g weight gain over subsequent weeks. The net energy cost of tissue synthesis, calculated from the difference between indirect and direct calorimetry was 3.2 ±1.1 kJ/g weight gain during the second week and 1.1 ± 0.1 kJ/g weight gain in the following weeks. A neonate who receives a caloric intake of 535 kJ · kg-1· 24 h-1and is growing at a rate of 17 g ·kg-1· 24 h-1will use 42% of the caloric intake for maintenance and thermoregulation, 6% for activity, 38% for the components of new tissue, 4% for tissue synthesis and 10% for loss in faeces and urine.