An increase in diaphragmatic electromyogram (EMG) recorded via oesophageal electrodes during positive pressure breathing has been reported to indicate an increase in the neural outflow to the muscle. However, using similar techniques, the size of compound muscle action potentials produced by supramaximal phrenic nerve stimulation also increased during positive pressure breathing. It is concluded that a change in EMG recording conditions rather than in central respiratory outflow may explain the increase in diaphragmatic EMG produced by positive airway pressure.