We have constructed a simple system for field anaesthesia by using a Farman entrainer and a semi-open circuit to convert a draw-over apparatus to a continuous flow air/O2 system. Compressed O2 was the driving gas for the entrainer; fresh gas (FG) delivered to the semi-open circuit was a mixture of O2, entrained air and anaesthetic vapour. The purpose of this study was to examine FG flow rate and CO2 rebreathing during intermittent positive pressure ventilation (IPPV). A non-rebreathing inflation valve (Laerdal) placed at the end of the expiratory (efferent) limb of the circuit vented both expiratory gas and excess FG. Ambient air IPPV was applied through the Laerdal valve from a self-inflating bag or ventilator. Since this circuit is functionally similar to a T-piece, the gas from the efferent limb (340 ml, containing FG) entered the lungs first. If tidal volume was larger than 340 ml the balance was ambient air. Minute ventilation of the lungs with efferent limb gas was defined as Veff. Respiratory gas was sampled at the endotracheal tube and the CO2 tension was measured with a NIHON-KOHDEN CO2 analyzer. Thirty-seven adult patients having intra-abdominal or pelvic surgery under general tracheal anaesthesia were studied. Four FG flow rates (5.7, 8.0, 9.3, and 10.4 L.min-1), corresponding to driving gas pressures of 40, 60, 80, and 100 mmHg, were introduced in random order. Although inspired CO2 was detected at FG flow rates of 5.7-9.3 L.min-1, there were no differences in PETCO2 among the four groups.(ABSTRACT TRUNCATED AT 250 WORDS)