The in vivo study of the pulmonary microcirculation, and its recruitment, is currently not common, although it may be of interest. The intrabreath analysis (IB) of the carbon monoxide (CO) and acetylene (C(2)H(2)) diffusion is used to study the transfer of CO (TLCO) and the pulmonary capillary blood flow (Qs), particularly during exercise. The evolution of the Qs during different stages of exercise has never been reported in healthy subjects. The authors measured the Qs and TL at rest and then during and after short bouts of exercise in 12 healthy subjects. The Qs increased from 5.6 L/min at rest to 13.8 L/min during exercise while the TLCO increased from 11 to 16.7 mmol/kPa/min. A linear relationship was found between the Qs and the TLCO, with Qs values close to those obtained with other techniques. The Qs returned to rest values more rapidly than the TLCO (probably because of the membrane factor). Pulmonary vascular recruitment can be easily studied in healthy subjects. This parameter may be important in the study in pulmonary vascular diseases.