Our analysis was aimed at determining the alveolar-capillary PO2 difference ([Delta]PAcO 2) during exercise. We hypothesized that values of the pulmonary nitric oxide diffusing capacity can be used to derive [Delta]PAcO 2. For this purpose, we analysed the pertinent data of three studies performed on 35 healthy, non- athletic non-smokers of similar age at seven different exercise intensities. Calculated mean values of alveolar-capillary PO2 difference aggravated from 2 mmHg at rest to 18 mmHg at a performance capacity amounting to 90 % of the maximum level. Regression analysis revealed y = 0.31x ? 0.0012x2 (n = 7, r = 0.999, P < 0.00001) where x represents the ratio (O2 consumption)/(maximal O2 consumption) and y stands for [Delta]PAcO 2, respectively. Due to the nonlinear increase of the alveolar-capillary PO2 difference with increasing O2 consumption, our model analysis confirms the opinion that pulmonary diffusion decreasingly determines maximal aerobic power.