EFFICIENT apparent photosynthesis and high rates of accumulation of dry matter in maize and other tropical grasses have been attributed to a lack of respiratory activity during illumination. Low CO2 compensation concentrations1,2, absence of a post-illumination CO2 burst3,4 and lack of CO2 release to CO2-free air5,6 support this contention. By contrast, many other species show sizable rates of photorespiratory release of CO2 (probably via glycolic acid oxidase) which, unlike dark respiration, are accelerated by concentrations of O2 above 2 per cent. In maize there is no effect of ambient O2 above 2 per cent on CO2 compensation values3,4 or on the uptake of net CO24,7,8. Moreover, low release of 14C from added glycolate-1-14C constitutes additional evidence against the occurrence of photorespiratory CO2 release in maize9. It has been suggested6 that very efficient internal trapping of photorespiratory CO2 in photosynthesis may account for these observations. The insensitivity of CO2 release to O2 concentrations between 2 and 21 per cent when net photosynthetic CO2 uptake was completely curtailed by DCMU [3-(3,4-dichlorophenyl)-1,1 dimethylurea] suggests8, however, the absence of a typical photorespiratory CO2 release process in maize. We describe here our observations showing that illuminated maize does exhibit a steady state O2 uptake component, which is stimulated by concentrations of O2 above 2 per cent.