Previous studies had indicated that maximum aerobic power (VO2 max) would be seriously impaired when HbCO levels were above 7% but was not altered if HbCO was around 2.7%. The present studies indicated that the critical level at which HbCO influenced VO2 max was approximately 4.3%. This was accompanied as in the above-noted studied with a reduction in total work time to the attainment of VO2 max. Two procedures to raise HbCO to appropriate levels were employed, i.e., a buildup wherin HbCO was incrementally increased by breathing ambient air containing 75 or 100 ppm CO and a bolus plus maintenance procedure. In the latter, HbCO was raised to the level attained in the buildup test by giving a "bolus" of CO followed by the continued inhalation of CO at a level to just maintain this level of HbCO regardless of the magnitude of the ventilation. Regardless of the mode of presentation, the decrement in VO2 max occurred at the same level of HbCO. These observations are of considerable significance, since it indicated that even low ambient levels of CO (23.7 ppm) would result in lowering maximum aerobic power if the individual had been previously exposed to CO such that the level was raised to this critical point.