Synopsis-Abstract The effect of a rapid 4% saturation elevation of carboxyhemoglobin (COHb) on maximal exercise and adaptation to this hypoxic stress over four hours was measured in six physically fit fire fighters. The volunteer subjects@ ages 25 to 39@ were randomly exposed in a double-blind experiment to either carbon monoxide (CO) or uncontaminated air twice a week for three weeks. On exposure days the subjects breathed 20@000 ppm CO for 45 seconds followed by 30 ppm CO for four hours. On control days CO-free air was substituted for CO. The subjects were exercised to exhaustion using the Bruce treadmill protocol immediately following the CO or air bolus and again after four hours. The rapid 4% saturation increase in COHb resulted in a decrease (P < .05) in the mean exhaustion times obtained immediately after the CO bolus (745.2 to 736.3 sec.). However@ this small exercise decrement was not consistently observed@ and in 16 of the 44 measurements@ the trained subjects performed better following CO exposure. The resulting COHb body burden also produced an increase in heart rate (177.7 to 179.3/min. at Stage 4)@ and increase in VE (127.2 to 131.4 2/M BTPS at Stage 4) and a decrease in P50 (27.5 to 26.3 mmHg@ post exercise samples). There was no evidence of adaptation to this hypoxic stress over the for hours or over the three week study period. There were no significant changes in systolic time intervals either pre- or post-CO bolus or pre- or post-exercise.