Thoracotomy is an uncommon procedure in horses but remains essential in a variety of cases of pleuropneumonia, pericarditis, thoracic trauma or diaphragmatic herniation, and for experimental thoracic and cardiac procedures. This study aimed at developing an experimental surgical procedure allowing access to the entire circumference of the heart and describing the effect of thoracotomy on pulmonary gas exchange in these horses. The study consisted of two arms, arm one (undergoing thoracotomy), was a terminal experimental study that included 11 Standardbred horses with experimentally induced (by tachypacing) atrial fibrillation. Arm two consisted of 6 Standardbred horses undergoing anesthesia for reasons unrelated to the present study. These horses functioned as controls. Anesthesia was induced using zolazepam and tiletamine. Anesthesia was maintained with isoflurane in 100% oxygen and ventilation with intermittent positive pressure (IPPV); no positive end-expiratory pressure (PEEP) was performed. Rib resection and pericardiotomy were performed for complete exposure of the entire circumference of the heart. Arterial blood samples were collected prior to, 5 and 30 minutes after puncture of pleura parietalis. In 10 horses, resection of the fifth rib was adequate for exposure of the heart. In one horse, removal of the sixth rib was also necessary. The duration of the surgical procedure (thoracotomy, pericardiotomy) was < 45 minutes. During a thoracotomy, PaO₂ decreased significantly (P <.05) from 291.8 ± 82.8 mmHg to 165.2 ± 73.5 mmHg but was not different from normal anesthetized controls. The PaCO₂ remained within normal limits. This surgical approach provided access to the entire circumference of the heart.