Five patterns of ventilation have been compared in dogs: (1) spontaneous ventilation at ambient pressure (SV); (2) intermittent positive pressure ventilation (IPPV); (3) spontaneous ventilation at 0.98 kPa positive airway pressure, delivered from a non-rebreathing circuit incorporating a 5-litre reservoir bag and fresh-gas flow of twice the minute volume (CPAP (bag)); (4) spontaneous ventilation at 0.98 kPa positive airway pressure, with the reservoir bag replaced by a weighted bellows (CPAP (bellows)) and (5) IPPV with 0.98 kPa positive end-expiratory pressure (CPPV). CPAP significantly decreased the rate of respiration compared with SV. CPAP (bellows) resulted in a significant increase in tidal volume. Mean oesophageal pressure, right atrial pressure, pulmonary wedge pressure and pulmonary artery pressure increased during IPPV, CPAP (bag), CPAP (bellows), and CPPV compared with SV. There were no significant changes in mean systemic arterial pressure, cardiac output, PaO2, PaCO2, CaO2, (CaO2-CVO2), PAO2-PAO2) or pulmonary venous admixture. Under the conditions of this study oxygen transport was not altered by positive airway pressure ventilation.