A total of seventy-four men divided into five groups performed a compensatory tracking task for 4 hr with group mean carboxyhemoglobin (COHb) levels of 0.9, 5.6, 6.1, 11.4 or 16.6%. In all but one group, COHb was formed in approximately 5 min by having the subject breathe a high concentration of carbon monoxide (CO) from a Douglas bag. An appropriate low level of CO in the exposure/test chamber maintained the elevated COHb for the 4 hr duration of the experiment. A slow COHb formation group, not exposed to CO during bag breathing, which was only exposed to low-level CO in the exposure/testing chamber, served as a positive control. Even at the highest dose, the increase in tracking error was small. The increased mean error score due to rapid formation was virtually the same as that due to slow formation.