The effects of different doses of orally administered sildenafil on pulmonary haemodynamics in dogs with pulmonary hypertension (PH) have not been documented in an invasive and quantitative manner. In this study, we examined the effects of oral sildenafil using a canine model of chronic embolic PH (CEPH). This CEPH model was created by repeatedly injecting microspheres through a catheter into the pulmonary artery under general anaesthesia at regular weekly intervals over several months. The CEPH dogs received 1, 2 or 4 mg/kg of sildenafil orally twice a day for seven days. Then, haemodynamic measurements including pulmonary artery pressure (PAP), systemic artery pressure (SAP), pulmonary artery wedge pressure (PAWP), right atrial pressure (RAP) and cardiac output (CO) were obtained after seven days of sildenafil administration via right heart catheterisation and oscillometric blood pressure measurements. Sildenafil was well tolerated in this study. Sildenafil administered at doses of 2 and 4 mg/kg significantly decreased systolic PAP compared with before administration. In addition, all doses of sildenafil significantly decreased the mean and diastolic PAP. Furthermore, 4 mg/kg of sildenafil significantly decreased PAP compared with 1 mg/kg. Sildenafil also significantly decreased pulmonary vascular resistance without notable changes in SAP or systemic vascular resistance. The PAWP, RAP and CO did not increase significantly at any doses. In conclusion, the oral administration of sildenafil to CEPH models decreased PAP in a dose-dependent manner.Copyright © 2020 Elsevier Ltd. All rights reserved.