1. A rapid method for the analysis of CO in expired air has been developed, which is suitable for use in studies of smoking.
2. The Bohr equation has been used to calculate the mean alveolar CO partial pressure (Pa,co).
3. The values of Pa,co obtained are highly correlated with direct measurements of venous carboxyhaemoglobin (r = 0·96).
4. The method will distinguish between populations of smokers and non-smokers, and can allow the changes of CO in a smoker throughout a 12 h period to be followed. It provides a measure of the dose of cigarette smoke (vapour phase) that results from smoking a single cigarette.