15 healthy adults (5 women, 10 men) have been exposed to each of four experimental conditions: rebreathing of air or carbon monoxide, and the smoking of two low-nicotine (0.3 mg) or medium-nicotine (1.2 mg) cigarettes. The dose of carbon monoxide selected (80 ml) had no effect on resting heart rate or electrocardiogram, nor did it modify the exercise heart rate, ventilation, oxygen consumption or electrocardiogram. Both types of cigarette produced variable changes in the blood carboxyhaemoglobin level, increments being greatest in heavy smokers, and least in former smokers, pipe and cigar smokers, Gains of carboxyhaemoglobin were less with low-nicotine than with medium-nicotine cigarettes. Both types of cigarette produced some increase of resting heart rate, but the smoking of two cigarettes was insufficient to modify the exercise heart rate, ventilation, or oxygen consumption. The electrocardiogram also showed no signs of ischaemia during either rest or effort. It is suggested that, for the light and moderate smoke, the 'safest' cigarette may be one with a low tar and a very low nicotine yield. On the present data, this would also induce only small increments of blood carboxyhaemoglobin levels.