This short Commentary has been limited to a few of several studies. They clearly indicate that the mechanisms involved in the effects of hypoxia and hyperoxia on the ventilatory control and particularly on theintegrative respiratory centres are far from being completely understood and are now believed to be more complex than what was postulated 40 years ago. Other mechanisms, which are beyond the scope of this short review, are currently the subjects of numerous studies which suggest that mediators or modulators are involved not only at the periphery but also at the level of the central nervous system. It must be emphasized that all these recent studies do not contradict at all, but rather help to explain, the early findings of Pierre Dejours concerning the participation of the peripheral chemoreceptors in the regulation and control of breathing.