Tolerance to hypoxic stress was examined in male C57BL mice of different ages through use of a hypoxic chamber (4% oxygen, 96% nitrogen). The basal survival times in a hypoxic chamber did not significantly differ between mice of different ages (6 wks, 8 wks, 10 months, 28 months). The pretreatment of antihypoxic prostaglandin I2 (PGI2) significantly prolonged the survival time in both adult (10-month-old and aged (30-month-old) mice, but the effect tended to be smaller in aged mice. The effect of sublethal hypoxia treatment on survival time in a hypoxic chamber was minimal in both adult and aged mice, but the effect tended to be smaller in aged mice. IP injection of sublethal doses of potassium cyanide (3.0 mg/kg) significantly prolonged the survival time in a hypoxic chamber when examined 10 min after the pretreatment in immature mice (6 wks), but in adult and aged mice, the survival time tended to be slightly shorter at that time than respective control values without pretreatment. The maximal survival time after KCN pretreatment was longest in immature mice and shortest in aged mice. The results suggest that the basal tolerance to hypoxic stress in mice is little affected by aging. However, the potential antihypoxic mechanisms appear to be progressively affected by aging.