Cardiopulmonary function in seven Beagles (under pentobarbital anesthesia) was determined before they were inoculated with Dirofilaria immitis larvae, at 6 and at 12 months after 50 infective larvae were inoculated, and at 7 and 12 months after adulticide and microfilaricide treatment was begun. Pulmonary hypertension (26.4 mm of Hg +/- 3.5 (SD)) was present at 12 months after the dogs were inoculated, as compared with base-line pulmonary arterial pressure (16.0 mm of Hg +/- 2.4), and was resolved within 7 months after initial adulticide treatment (19.8 mm of Hg +/- 1.4). Pulmonary artery pressure response to hypoxia (33.3 mm of Hg +/- 2.4) at 12 months after inoculation (vs base line of 25.3 mm of Hg +/- 2.2) and to isoproterenol infusion (37.6 mm of Hg +/- 4.8) at 12 months (vs base line of 14.3 mm of Hg +/- 1.7) revealed an accelerated pulmonary hypertensive response. This pulmonary hypertensive response to isoproterenol infusion (17.7 mm of Hg +/- 1.4) at 7 months after treatment resolved following treatment. The pulmonary hypertensive response to hypoxia was still present (32.5 mm of Hg +/- 3.0) 7 months after treatments. Resolution of pulmonary hypertension reinforces the clinical decision to treat dirofilariasis in an effort to prevent further development of pulmonary hypertension, as well as to permit resolution of this clinical sign.