CO2 single breaths have been performed in 7 men and 7 women in conditions of normoxia (FICO2 congruent to 0.13; FIO2 congruent to 0.21; FIN2 congruent to 0.66) and of hyperoxia (FICO2 congruent to 0.13; FIO2 congruent to 0.87). Ventilatory responses of the subjects and modifications of breathing pattern in the course of the CO2 tests were also explored in the two conditions. The results (mean +/- SEM) show that, whatever the oxygenation, men and women exhibit the same ventilatory response during a CO2 test from a qualitative point of view but with a smaller intensity in women (men: 0.37 +/- 0.088 LBTPS.min-1.Torr-1; women: 0.15 +/- 0.025 LBTPS.min-1.Torr-1; p less than 0.05). Considering men and women together, CO2 tests induced an increase of minute volume VE (p less than 0.001), VT (p less than 0.01) and rate of breathing (NS) but this response is decreased in hyperoxic conditions (p less than 0.05) mainly in men (men: 0.19 +/- 0.043 LBTPS.min-1.Torr-1; women: 0.11 +/- 0.023 LBTPS.min-1.Torr-1). These results show that sensitivity to transient hypercapnia and its interaction with hyperoxia are weaker in women than in men.