Pathogen-free OF1 male and female mice of six different ages (between 31 and 387 days), grouped by 10, sex-separated and synchronized by a light (100 lux)-dark 12:12 alternation, were submitted to an acute LD50 carbon monoxide intoxication during the light period during which respiratory and motor activities are at a minimum. For this range of ages carbon monoxide survival is not significantly influenced by age and is significantly (P less than 0.001) less so in males than in females. Resistance to the acute carbon monoxide challenge seems more related to factors influencing the longevity of this mouse strain where grouped males are particularly aggressive than to respiratory exchanges (VCO2), heart frequencies or body temperature.