Objective: To evaluate the cardiorespiratory effects of a 7 degrees reverse Trendelenburg position (RTP) in anaesthetized horses. Study design: Randomized, non-blinded clinical trial. Animals: A total of 125 horses undergoing elective surgery in dorsal recumbency. Methods: Horses were allocated to one of three weight classes and assigned to be positioned either on a horizontal table or on a table in 7 degrees RTP, according to a randomized block design. In all horses, anaesthesia was maintained with isoflurane in oxygen and a constant rate infusion of romifidine. All horses were mechanically ventilated throughout anaesthesia, and routine cardiovascular monitoring and arterial blood gas analysis were performed at 15-minute intervals and relevant variables calculated. Data from the first 60 minutes of anaesthesia were compared between both positions using a mixed model analysis of variance. Results: A significant interaction was found between position and weight class for the alveolar to arterial oxygen tension gradient and F-shunt: these variables were lower in RTP than in horizontal position in the two lowest weight classes and vice versa in the highest weight class. Arterial oxygen tension and oxygenation indices were significantly worse in the horses in the higher weight classes. Conclusions and clinical relevance: A 7 degrees RTP did not result in clinically relevant changes in gas exchange or cardiovascular function. Horses with a higher body weight are at increased risk for hypoxaemia during anaesthesia in dorsal recumbency.