The pulmonary diffusing capacity (DLCO SB) and its two components, the capillary blood volume (Vc) and the diffusing capacity of the membrane (DMCO), expressed in absolute values and per litre of alveolar volume (VA'), were measured at rest and on exercise in healthy male smokers and nonsmokers of similar age and height, and with identical values for haemoglobin and spirographic data. DLCO, DLCO/VA', DMCO and DMCO/VA' are significantly lower in smokers, at rest and on exercise; the decrease in Vc and thetaVc/VA' in smokers at rest is due to a higher level of carboxyhaemoglobin. The decrease of DLCO, DLCO/VA', DMCO and DMCO/VA' is apparently not due to carboxyhaemoglobin or distributional factors but to anatomical lesions, probably of emphysematous nature, altering the pulmonary membrane. Formulas predicting DLCO, DMCO, Vc, DLCO/VA', DMCO/VA' and thetaVc/VA' in terms of age and height were established in smokers and in nonsmokers.