Carbon dioxide elimination during apnoea was measured in sixty patients undergoing diagnostic bronchoscopy. Paralysis was maintained using suxamethonium after thiopentone induction of anaesthesia. In forty patients a 50 per cent mixture of nitrous oxide and oxygen was insufflated by nylon catheter in the trachea at a rate of 10 l./min. The mean quantity of carbon dioxide eliminated was approximately 20 ml/min (range 0–0.8 per cent). In twenty patients without insufflation the corresponding rate was approximately 6 ml/min (range 0–0.25 per cent). The relationship of this finding to the known effect of the heartbeat in producing a small amount of alveolar ventilation is discussed.