Two hundred and three male patients undergoing oral surgery were monitored for abnormalities of cardiac rhythm. Eight per cent of a smaller number of patients (seventy-six) developed ectopic rhythms during induction of anaesthesia and in half of all patients nodal rhythm was seen. Twenty-three per cent developed ectopic rhythms in response to surgery. In three per cent of patients these were of the “chaotic” type. Most of the ectopic rhythms were nodal (supraventricular) in origin. Stimulation of the sympatho-adrenal system via the fifth cranial nerve in the presence of halothane anaesthesia is considered to be the main factor in the production of these dysrhythmias. Their significance is discussed