The heart rate and blood pressure at rest and during exercise and physical work capacity (PWC) at different heart rates were measured in 14 healthy smokers after air breathing, after a 1100 ppm CO- air mixture breathing and after cigarette smoking. After air breathing the mean COHb saturation was 3.1 (0.6 SD) %, after CO inhalation it was 9.9 (1.6)% and after smoking 9.8 (1.8)%. The heart rate at rest was unchanged after CO, but smoking caused an increase as compared with both air breathing and CO inhalation (p less than 0.001). The blood pressure was not affected by CO or smoking either at rest or during exercise. The physical work capacities at heart rates 130, 150 and 170 beats per minute decreased both after CO inhalation and after smoking. The change was greater after tobacco smoking. The greatest decrease in circulated mean maximal work was after CO inhalation. The result deviates from PWC 170 and could be taken to indicate that tobacco smoking also acts as a stimulant during exhaustive work.