Energy balance studies were done during 10-29 days on 15 immature infants of mean birth weight 1581 g. Mean gross energy intake was 757 kJ/kg (181 kcal) and 79% of this was retained, so that metabolizable energy was 602 kJ/kg (143 kcal). Mean resting metabolic rate was 244 kJ/kg (58.1 kcal), and it increased with advancing maturity. Minimum resting metabolism averaged 199 kJ/kg (47.5 kcal). Energy expended in activity increased with maturity, but amounted to less than 17% of the total energy turnover. Postprandial metabolism caused the mean VO2 to rise by 17% in the hour after a feed, and during 24 hr resulted in consumption of energy equivalent to about 10% of the resting metabolism. Stored energy amounted to 230 kJ/kg (55 kcal) and was linearly related to weight gain (r = 0.92). Energy cost of weight gain was 24 kJ/g (5.7 kcal) and energy stored in new tissue was 16.8 kJ/g (4.0 kcal). Maintenance energy requirement at zero growth rate was about 270 kJ/kg (64 kcal).