Abstract 1. 1. Respiratory rates of workers of the leaf cutting ant Atta sexdens rubropilosa were measured at different oxygen pressures, at 25°C. 2. 2. In experiments where different ants were used at each of the oxygen pressures, respiration was regulated down to 70.8 mmHg. 3. 3. When the same ants were submitted in sequence to declining pO 2, the “oxygen dependence indexes” (Tang P. S. (1933) Quart. Rev. Biol. 8, 260–274) also suggested a good regulatory capacity. 4. 4. The results are discussed in terms of the variation of the partial pressures of O 2 and CO 2 that the ants probably encounter when wandering to and from the nest to forage, and when performing their heavy tasks (leaf transport, offspring and fungus care). 5. 5. CO 2 rise and O 2 fall, from ants' respiration inside the ant hill, may act as the factors that, in a reflex way, keep the spiracles open and increase ventilation and the frequency of CO 2 emission to keep the oxygen supply adequate to face the energetic demand of the routine level of activity of the workers, when passing from nonnoxia (air) to hypoxia (in nest galleries).