Respiratory water loss (RWL), oxygen consumption and carbon dioxide production were measured in ten fullterm infants on their first day after birth, using an open flow-through system with a mass spectrometer, specially equipped with a water channel, for analysis of gas concentrations. Measurements were made both with the infant asleep and during different levels of motor activity. The ambient temperature was maintained at approximately 32.5 degrees C and the ambient relative humidity at 50%. RWL increased from 4.2 +/- 0.7 (SD) mg/kg min when the infant was asleep to 6.3 +/- 1.0 mg/kg min when he or she was awake but calm; with increasing activity there was a further increase in RWL. The oxygen consumption increased from 5.4 +/- 0.9 (SD) ml/kg min during sleep to 6.9 +/- 0.8 (SD) ml/kg min when awake, and also increased further with increasing activity.