Children with congenital complete atrioventricular block (AVB) do not often report exercise intolerance due to compensatory mechanisms. The effect of cardiac remodeling being absent and present after AV block on exercise capacity has not yet been reported. We studied the cardiorespiratory changes due to exercise at three different timepoints after inducing AV block in dogs. Mongrel dogs (n=8) were placed on a treadmill with a 10% incline in a custom-build closed chamber and performed a moderate exercise protocol (10 minutes at a speed of 6 km/h). Dogs ran at sinus rhythm (SR), at two days after AV block (AVB2d) lacking cardiac remodeling, and at three and six weeks of chronic AV block (CAVB) remodeling (CAVB3 and CAVB6, respectively). ECG tracings were recorded via electrodes on the back. Stroke volume and cardiac output were recorded via a thoracic electrical bioimpedance-based device (PhysioFlow®). pO2, pCO2, SpO2 and pH levels were determined from venous blood samples. O2 consumption (VO2) and CO2 production (VCO2) were measured within the chamber. All dogs completed the exercise protocol at SR, CAVB3 and CAVB6, but 5/8 dogs failed at AVB2d (average run duration: 7min23sec, SD: 2min38sec), due to ventricular tachycardia and/or staggering. Changes in heart rate, stroke volume and cardiac output upon exercise were not different between the timepoints after AV block. At AVB2d, pO2 was lower (3.3±1.1 vs. 5.2±0.7 kPa (SR), p<0.01), the SpO2 was decreased (31±16 vs. 67±7 % (SR), p<0.001), the pCO2 was higher (7.3±1.4 vs. 5.5±0.3 kPa (SR), p<0.01), and the pH was decreased (7.23±0.06 vs. 7.36±0.02 (SR), p<0.001) at 2min exercise. Similar values were found at 5min exercise e.g., for pCO2 (6.8±1.8 vs. 5.1±1.3 kPa (SR), p<0.05) and pH (7.22±0.08 vs. 7.37±0.03 (SR), p<0.001). Venous blood parameters were not different between SR, CAVB3 and CAVB6 at rest and during exercise. At the end of the run (at steady state), VO2 and VCO2 are lowest at AVB2d with a 10% and 5% decrease, respectively, compared to SR (p=0.05 and 0.33). Dogs without remodeling after AV block have a reduced exercise capacity, which seems to be reflected in changes in cardiorespiratory parameters. Remodeling after chronic AV block improves the exercise capacity in dogs.