We have used in vivo microscopy to show that airway hypoxia caused pulmonary capillary recruitment. To determine whether the recruited capillaries add to the surface area for gas exchange, we measured the diffusing capacity of the lung for carbon monoxide before and after inducing recruitment with hypoxia. Diffusing capacity increased during hypoxia. However, some increase in diffusing capacity was expected, since there were fewer oxygen molecules to compete with carbon monoxide for hemoglobin-binding sites. To determine the effect of capillary recruitment alone on diffusing capacity, we held hypoxia constant while infusing a vasodilator to diminish recruitment. Diffusing capacity decreased concomitantly as recruitment diminished. These results indicate that our microscopic observations reflect a widespread recruitment that increases the gas exchange surface area.