We have measured oxygen uptake (VO2) and carbon dioxide production under steady-state conditions in 14 conscious volunteer subjects breathing from the Mapleson A system at different fresh gas flow rates, by collecting gas vented from the breathing system. Mixed vented gas concentration was measured with a mass spectrometer and volume with a water spirometer. Gas exchange and total ventilation were measured with subjects under resting conditions breathing room air, and then whilst breathing from a Magill system. Mean values of VO2 at mean fresh gas flow rates 1.43, 0.88, 0.67 and 0.49 times the resting total ventilation of the subject, were 3.30, 3.30, 3.34 and 3.56 ml min-1 kg-1 (STPD), respectively. Rebreathing occurred at ratios of 0.67 and 0.49. We were unable to demonstrate any increase in VO2 in the Mapleson A system with rebreathing.