The pulmonary diffusing capacity for carbon monoxide, Dco, was estimated in normal subjects, using both a single breath technique (DcoSB) and a rebreathing technique (DcoRB). In order to measure CO by mass spectrometry, the stable isotope C(18)O was used. In three normal subjects Dco was measured at different lung volumes at rest, and at different levels of bicycle exercise. DcoRB was found on the average 30% higher than DcoSB when compared at the same mean lung volume, and both increased slightly during exercise. The advantages and drawbacks of the rebreathing method are critically discussed.