Rationale: Previously we have shown, using contrast echocardiography, that intra-pulmonary arteriovenous pathways are inducible in healthy humans during exercise, however this technique does not allow for determination of arteriovenous vessel size or shunt magnitude. Objectives: The purpose of this study was to determine if large diameter (>25 µm) intra-pulmonary arteriovenous pathways are present in the dog, and whether exercise recruits these conduits. Methods: Through the right fore-limb, 10.8 million 25 µm stable isotope-labeled microspheres (BioPAL, Inc.^TM) were injected either at rest (n=8) or during high-intensity exercise (6-8mph, 10-15% grade, n=6). Systemic arterial blood was continuously sampled during, and for three minutes after injection. Following euthanasia, tissue samples were obtained from the heart, liver, kidney and skeletal muscle. In addition, 25 and 50 µm microspheres were infused into four isolated dog lungs that were ventilated and perfused at constant perfusion pressures similar to exercise. Measurements and Main Results: Blood and tissue samples were commercially analyzed for the presence of microspheres (BioPAL, Inc.^TM). No microspheres were detected in the arterial blood or tissue samples from resting dogs. In contrast, five of six exercising dogs showed evidence of exercise-induced intra-pulmonary arteriovenous shunting, as microspheres were detected in arterial blood and/or tissue. Furthermore, shunt magnitude was calculated to be 1.4±0.8% of cardiac output (n=3). Evidence of intra-pulmonary arteriovenous anastomoses 50µm was also found in three of four isolated lungs. Conclusions: Consistent with previous human findings, these data demonstrate that intra-pulmonary arteriovenous pathways are functional in the dog and are recruited with exercise.