SummaryWe have developed a method for measuring oxygen consumption (&OV0312;O2) in preterm infants in their normal incubator environment over prolonged periods. The results of measurements made over 24 h in 18 infants are presented. In normally grown infants, the mean &OV0312;O2was 9.66 ± 1.25 liters/kg•24 h (SD) (6.71 ± 0.87 ml/kg•min), and in small for gestation infants it was 10.09 ± 1.21 liters/kg•24 h (7.00 ± 0.84 ml/kg•min). During the 24‐h measurements, the highest mean &OV0312;O2during 3 consecutive h was 7.75 ± 0.89 ml/kg•min and the lowest was 5.95 ± 0.92 ml/kg•min. The difference between the highest and the lowest values was significant (p< 0.001). There is room for considerable error if short term measurements are assumed to represent values over a whole day. “Short” measurements should be made over at least 6 h.Abbreviations&OV0312;O2, oxygen consumption rateMR, metabolic rateME, metabolizable energy intakeRQ, respiratory quotient