There has been much discussion concerning the relative merits of general anaesthesia in the upright and horizontal postures for dental patients.Bourne' has drawn attention to the vulnerability of the brain should hypotension occur during such a procedure with the patient seated upright. He has suggested that 'fainting' may occur and pass unnoticed during induction of anaesthesia. There is considerable material available in the literature on the cardio-respiratory responses under such circumstances,'-" but relatively little has been published on the effects of nitrous oxide: halothane with concentrations of oxygen in excess of 21% delivered to patients in the sitting position.In order to obtain information as to the likely cardio-respiratory responses during anaesthesia under the latter conditions we have carried out a study on dental outpatients receiving general anaesthesia for exodontia. MethodCardiovascular and respiratory responses were studied on fifty-three healthy patients, undergoing dental extractions as outpatients, under general anaesthesia. No premedication was given but all patients were checked for their fitness for surgery and anaesthesia by the dental surgeon and the anaesthetist. A pre-operative reading of the blood pressure and pulse rate was made in the examination room.The patients were placed in sitting up position in the dental chair and anaesthesia was administered through a nosepiece. A MIE Salisbury dental anaesthetic machine was used to administer 70/30 nitrous oxide/oxygen, supplemented by halothane in a dosage not exceeding 2%, delivered by a fluothane mark I V vaporizer recently checked for accuracy.Before induction of anaesthesia electrocardiogram (ECG) leads were fitted to the left and the right arm of the patient. An XE-350 earpiece was fixed to the ear lobe, and was protected from light for the measurement of oxygen saturation. The blood pressure cuff was fixed to the arm, and a strain gauge placed around the patient's chest to sense respiratory rate and depth.During anaesthesia the systolic blood pressure was measured every 30 s using the standard mercury sphygmomanometer and palpation of the radial artery.The XE-350 earpiece was attached to an X-350 oximeter, and the changes in the oxygen saturation were recorded every 60 s (this method has been reported elsewhere12).The ECG was recorded continuously for the whole operating time, as well as the immediate pre-operative period, and the interval from the end of anaesthesia to the time of awakening. The various events of the operative and anaesthetic sequences were recorded: the onset of induction, the placing of prop and pack, the extraction of teeth, the change of position of the prop and the variations in the percentage of halothane. Observation was discontinued when the eyes opened.