BACKGROUND: Sildenafil improves autonomic dysfunction caused by pulmonary hypertension (PH) in humans, but its effect is unknown in dogs with PH. This prospective study aimed to evaluate the autonomic nervous system function of a canine model of chronic embolic PH (CEPH) and the autonomic nervous system function of a canine model of CEPH in which sildenafil was administered. METHODS: This study used five clinically healthy female beagle dogs. Evaluation parameters included hemodynamic parameters, heart rate (HR) and heart rate variability (HRV). Each evaluation parameter was compared before and after creating the CEPH model (before, BL; after, CEPH(BL)) and between the CEPH(BL) model and after the administration of sildenafil (1 mg/kg, BID) in the CEPH model dogs (CEPH(Sil)). RESULTS: In the CEPH(BL) model, the hemodynamic parameters indicated cardiac hypofunction, and HR was significantly increased and HRV was significantly decreased compared with BL. Further, in the CEPH(Sil) model, the hemodynamic parameters suggested improvement in cardiac function, and HRV was significantly increased. CONCLUSIONS: From the results of the CEPH model dogs, autonomic dysfunction was shown to occur in PH dogs. In addition, the administration of 1 mg/kg of sildenafil to CEPH model dogs may improve autonomic dysfunction.