Sildenafil is a phosphodiesterase type 5 (PDE-5) inhibitor found in corpus cavernosum and pulmonary smooth muscles. PDE-5 is an enzyme that induces cGMP degradation, and its inhibition results in the appearance of cGMP increase in tissue. It causes accumulation of nitric oxide in tissue, relaxation of smooth muscles and vasodilatation. It is rapidly absorbed following oral administration but its absorption is delayed when given with fatty foods. Its half-life is 6 hours for animals and 4 hours for humans. It is primarily metabolized by hepatic CYP 3A4. Cimetidine, Erythromycin, HIV protease inhibitors raises Sildenafil plasma levels. Co-administration of alpha adrenreceptor antagonists and sildenafil may result in life-threatening hypotension. The use of nitrate is contraindicated with sildenafil. Sildenafil was used 3 times a day initially. We observed decrease in cough, increse in apetite and regression in the existing mild ascites. Sildenafil P.O caused vomiting after the first 2 doses at half an hour and an hour apart. Vomiting was not repeated with antiemetic drugs used for 2 days. Our patient, who remained stable for about 1 month, returned to our clinic with the complaint of fatigue and Sildenafil decreased to 2 tablets per day. Symptoms of hypotension decreased. During this period, hypotension was seen again when patients accidentally given 3 tablets a day. Clinical symptoms decreased and stability was observed with regular drug use. After 3 months, the clinical symptoms began to intensify and the owners decided to let the animal to be operated for PDA.