The fox has one of the highest mass specific rates of maximal oxygen consumption (VO2max/Mb) that has been measured, yet its specific pulmonary diffusing capacity (DLO2/Mb, measured morphometrically) is similar to that of most mammalian species. It achieves a high O2 flux per unit DLO2 with a large partial pressure difference driving O2 diffusion from alveolar gas to capillary blood (PAO2-PbO2). This paper explores the mechanisms that the fox utilizes to achieve this large pressure difference and the extent to which it exploits its structural diffusing capacity. Foxes were exercised on a treadmill at maximal rates of O2 uptake. The following parameters were measured or calculated: arterial and mixed venous PO2, PCO2, pH and O2 concentration of the blood, cardiac output, hemoglobin concentration and O2 equilibrium curve of the blood, and morphometric estimates of pulmonary capillary volume and pulmonary diffusing capacity for O2. These data were used to calculate pulmonary capillary transit time and the time course of the change in O2 concentration and PO2 of the blood as it transits the lung. The fox has a morphometric pulmonary diffusing capacity of 0.098 ml O2.sec-1.mm Hg-1.kg-1. At VO2 max (3.6 ml O2.sec-1.kg-1) the fox hyperventilates, resulting in a high PAO2 (124 mm Hg); it also maintains a low PbO2 (88 mm Hg) by having a short transit time (0.13 sec) due to a high specific cardiac output (25 ml.sec-1.kg-1). Our calculations indicate that at VO2max the fox uses almost all of the pulmonary capillary transit time for O2 equilibration, in contrast to other species.