The use of assisted ventilation is required in anesthetized reptiles as their respiratory drive is lost at surgical depths of anesthesia. The minute volume of the assisted ventilation influences arterial blood gases and acid-base regulation. Meanwhile, the ventilatory pattern may also affect hemodynamics in chelonians, which, given their large capacity for cardiac shunts, may impact the efficacy of the ventilation in terms of gas exchange. Hence, there is a need for primary information on the influence of assisted ventilation on chelonian physiology, and we, therefore, performed a randomized study into the effects of recumbency and maximum airway pressure on pressure-cycled ventilation in nine female Trachemys scripta scripta. Pronounced effects of ventilation pressure on arterial PCO₂ and pH regardless of recumbency were revealed, whilst dorsal recumbency led to a larger Arterial-alveolar (A-a) O₂ difference, suggesting compromised pulmonary gas exchange. Plasma [Na⁺] and [K⁺] balance was also significantly correlated with maximum airway pressure. Computed tomography (CT) scanning at a range of end-inspiratory pressures and ventral and dorsal recumbencies in eight T. scripta scripta showed that lung volumes increase with maximum ventilatory pressure, while recumbency did not influence volume at pressures above 5 cmH₂O. Static compliance of the lungs was influenced by recumbency at neutral pressures. In conclusion, dorsal recumbency reduces pulmonary efficacy during positive pressure ventilation and tends to lower lung volume when ventilation is not provided. However, lung volumes and function - even in dorsal recumbency - can be adequately supported by assisted ventilation, and an end inspiratory pressure of 10 cmH₂O at 4 breaths min⁻¹ provided the most physiologically appropriate ventilation of anesthetized T. scripta scripta.