The present study examined the effect of hyperoxia on oxygen uptake (VO(2)) and on maximal oxygen uptake (VO(2max)) during incremental exercise (IE) and constant work rate exercise (CWRE). Ten subjects performed IE on a bicycle ergometer under normoxic and hyperoxic conditions (30% oxygen). They also performed four 12-min bouts of CWRE at 40, 55, 70 and 85% of normoxic VO(2max) (ex1, ex2, ex3 and ex4, respectively) in normoxia and in hyperoxia. VO(2max) was significantly improved by 15.0 (15.2)% under hyperoxia, while performance (maximum workload, W(max)) was improved by only +4.5 (3.0)%. During IE, the slope of the linear regression relating VO(2) to work rate was significantly steeper in hyperoxia than in normoxia [10.80 (0.88) vs 10.06 (0.66) ml x min(-1) x W(-1)]. During CWRE, we found a higher VO(2) at ex1, ex2, ex3 and ex4, and a higher VO(2) slow component at ex4 under hyperoxia. We have shown that breathing hyperoxic gas increases VO(2max), but to an extent that is difficult to explain by an increase in oxygen supply alone. Changes in metabolic response, fibre type recruitment and VO(2) of non-exercising tissue could explain the additional VO(2) for a given submaximal work rate under hyperoxia.