The effect of exercise on the increase of exhaled CO in smokers compared to non-smokers has not been clarified yet. In this study we compared the dynamics of exhaled CO before, during and after exercise between smokers and non-smokers. A group of 8 smokers and a group of 8 non-smokers underwent a bicycle exercise in a ramp fashion to near maximum intensity. Ventilation and gas exchange, and CO exhalation were continuously measured every 30-s before, during and after the exercise. The fraction of CO (F (CO)) in the exhaled air decreased gradually, but the total amount of exhaled CO (V(CO)) increased in a linear manner during the ramp exercise, and F (CO) and returned to the pre-exercise level within several minutes after exercise in all subjects. A linear relationship was observed between V (O(2)) and V (CO) and between V (E) and V (CO) in both the whole period of measurement and during the ramp exercise period in all subjects. However, the at V (CO) 0 W, the peak V (CO) and the slope coefficients in the regression equation between V (CO) and V (O(2)) and between V (CO) and V (E) in the ramp exercise as well as the entire periods of measurement were significantly higher in smokers compared with those in non-smokers, and these were correlated with the number of cigarettes smoked per day. It was concluded that CO exhalation increases linearly with the increase of V (O(2)) and V (E) during exercise, and habitual smoking shifts these relationships upward depending on the number of cigarettes smoked daily.