Pulmonary arterial hypertension is characterized for an increasing of pulmonary vascular resistance, which in results pressure and volume overload in the internal part of the right ventricle with consequent hypertrophy and dilation of this member, leading to right ventricular failure and death. Among the pathological mechanisms involved, we can include pulmonary endothelial dysfunction, which leads to a loss in production of vasodilators such as nitric oxide and prostacyclin and a high expression of vasoconstrictors such as endothelin. Commonly occurs secondary to cardiopulmonary disease, but can report to primary or idiopathic. The secondary clinical signs of pulmonary arterial hypertension are often silent, however, during exercises or in severe pulmonary hypertension, patients will present easy tiredness, intolerance to exercises, syncope and right heart failure. The diagnosis is obtained by direct measurement in the pulmonary artery, or as an alternative, the pressure gradient can be estimated using the echodopplercardiography. The treatment of pulmonary arterial hypertension is limited and typically directed to the basic cardiopulmonary disease, Sildenafil citrate has been successfully used, it's a highly selective inhibitor of phosphodiesterase type 5, the predominant isoform in the lung, promoting an increase in concentrations of oxide Nitric the pulmonary circulation resulting in vasodilatation. This report refers to two dogs: a British cocker spaniel, 11 years old, male (dog 1) and dachshund, 12 years old, (dog 2), the first case was forwarded to Veterinary Hospital Anhembi Morumbi with a history of severe dyspnea, intolerance to exercise, dry cough and severe cyanosis; the second case was with moderate dyspnea, intolerance to exercise, dry cough for 8 months and moderate ascites. Physical examination, chest X-ray, electrocardiography and echodopplercardiography were made, which concluded the diagnosis of moderate pulmonary hypertension (52 mmHg) secondary to chronic lung disease (dog 1) and severe (75 mmHg) secondary to endocardiosis mitral valve (dog 2) The conventional treatment of the underlying disease associated with sildenafil citrate (1 mg, kg 1 every 12 hours, 1 and dogs) was done. In the first appointment, after 7 days of the medication, dog 1 has showed significant improvement of dyspna and cyanosis and moderate improvement in cough. Dog 2 showed an improvement of general condition, dyspnea and physical activity. The improvement of the animals was maintained in both of them, after 30 days.