Cardiopulmonary function in Beagles that had been infected 18 months previously with 100 infective Dirofilaria immitis larvae was studied before and after an exercise program. The treadmill exercise program lasted for 2.5 months at 5 days per week until the dogs were running for 20 minutes at 11.3 km/hr up a 9% grade. Although the Beagles had mild pulmonary hypertension (mean pulmonary arterial pressure before training of 26.8 +/- 2.2 [SD] mm of Hg) and radiographic signs of cardiopulmonary disease, the dogs tolerated the program without signs of congestive heart failure or further development of pulmonary hypertension. Pulmonary arterial pressure was not increased during the 20-minute exercise period when measured at the completion of the 10-week exercise program.