Expiratory discharges in cervical afferent vagal fibres during spontaneous respiration were observed in anesthetized animals (17 rabbits, 4 cats and 2 monkeys). The percentages of such units among the total observed fibres was 11% in rabbits, 5% in monkeys, 2% in cats. All the experiments were done after section of the recurrent laryngeal nerve and the abdominal branches of the vagus nerve. Changing the intraesophageal pressure from +15 mm Hg to -25 mm Hg by injection or suction of air into or out of the esophagus, of which the abdominal end had been ligated, did not affect the expiratory discharges significantly suggesting that the receptors were not in the esophagus. Injection of air into the lungs to elevate the intratracheal pressure to 5 mm, 10 mm or 15 mm Hg could not excite such receptors. Collapse of the lungs caused by artificial pneumothorax produced continuous discharges in such fibres. Inflation of collapsed lungs by an artificial respiration pump stopped the sustained discharges immediately. The average conduction velocity of the afferent fibres was 25.5 m/s. It seems that this is a type of slowly adapting, low threshold pulmonary receptor with medium sized afferent fibres. The adequate stimulus of such receptors is deflation of the lungs. The possible advantage of participation of such receptors, in addition to the pulmonary stretch (inflation) receptors, in regulation of normal respiration is discussed in the light of the concept of 'paired receptors'.