The effect of alveolar volume (VA) on diffusing capacity for carbon monoxide (DL), membrane conductance (Dm) and pulmonary capillary blood volume (Qc) was investigated in 39 normal volunteers to study alveolar membrane expansion and capillary volume recruitment. DL/VA was related to alveolar volume breathing air and 95% oxygen respectively. Both relations appeared to be linear with a negative slope and were used to calculate Dm and Qc as a function of VA. The relation between Dm and VA resulted in: Dm = kVAx, where x characterizes the kind of membrane expansion with increasing alveolar volume, when we assume Dm = kA/delta. In this equation, A is the membrane area and delta the membrane thickness. In 36% of our subjects, x was nearly 0.67, which corresponds to an isotropic expansion of diffusion area with alveolar volume without changes in delta. In 41%, x was between 0.67 and 1. We hypothesized that, in these subjects, either some decrease of delta or some recruitment of alveoli was superimposed on the area increase according to x = 2/3. In recruitment, a proportional increase of diffusion area with alveolar volume is assumed, which means x = 1. Subjects over the age of 50 years (n = 12) showed greater variation in the value of x, which was greater than 1.3 in three and less than 0.6 in a further three. The relation between Qc and VA was best described by a second order polynomial, characterized by a maximum above 50% of VAmax. With lung expansion, either a recruitment of capillaries or a better contact between blood and air could occur up to that maximum. Where Qc was decreasing with further rise of VA, we assumed compression of capillaries by stretching of pulmonary tissue.