Maximal oxygen uptake (VO2 max) was measured in 180 children during exhaustive work on a bicycle ergometer. The material comprised 12 blind boys and 11 blind girls (8-14 years) as well as 82 normal boys and 75 normal girls (8-17 years). VO2 max increased linearly with age in all four groups. In normal girls mature values were reached at the age of 14 years. Normal boys had significantly higher values than normal girls and their VO2 max increased faster with age. No sex differences in VO2 max were found in blind children. Normal children had significantly higher values than the blind. VO2 max/kg was uninfluenced by age in three of the groups: 55, 45 and 37 ml/min/kg in normal boys, blind boys and blind girls, respectively. In normal girls VO2 max/kg decreased with age from 51 to 42 ml/min/kg. Significant sex differences were found in both normal and blind children. VO2 max/kg in blind boys was 82% of that of normal boys, while blind girls had significantly lower values than normal girls. Most of these differences were established already at the age of 8 years. It is concluded that the differences in maximal oxygen uptake between normal and blind children are to a high degree due to different levels of physical activity during early childhood.