Oxygen consumption (VO2, ml min-1) and carbon dioxide elimination (VCO2, ml min-1), minute ventilation (VE), tidal volume (VT), rate of ventilation (f) and end-tidal carbon dioxide concentration (E'CO2%) were measured in 38 infants and children (body weights 3.6-25 kg). Four children (body weight less than 5 kg) had congenital heart malformations and were studied during controlled mechanical ventilation, whereas the remainder (n = 34) who were healthy, breathed spontaneously. Anaesthesia was maintained with oxygen in air (FIO2 0.45) and halothane through a non-rebreathing circuit. Minute ventilation was measured by pneumotachography, E'CO2 with an in-line infra-red carbon dioxide meter and gas concentrations with a mass spectrometer. There were no differences in VO2 and VCO2 between children with and without heart disease. VO2 was related to body weight by the equation: VO2 = 5.0 x kg + 19.8 (r = 0.94) and VCO2 to body weight by the equation: VCO2 = 4.8 x kg + 6.4 (r = 0.94). There were no differences between VO2 or VCO2 before and after the start of surgery. In 11 of 21 patients weighing less than 10 kg, a reduced VCO2 was noted, giving respiratory quotients of less than 0.7. It is speculated that this age-dependent variation of VCO2 may result from partial inhibition of lipolysis in brown adipose tissue produced by halothane.