A case of cardiac arrest is presented which was caused by improper connection of a modified Mapleson D circuit (Bain breathing circuit). Excessive enternal deadspace was created by interchanging the gas inflow line and the attachment for an airway pressure manometer. This resulted in a marked respiratory acidosis, clinically undetected until the concomitant hypoxia produced a severe cardiac depression and arrest. The sequence of events was reproduced in a dog under comparable anaesthetic conditions. In order to avoid this error it is recommended to permanently fuse the connecting piece placed in the circuit with the tubing leading to the airway pressure manometer of the respirator.