In 50 patients, randomly assigned to two groups. who were submitted exclusively to extra-abdominal and extra-thoracic surgery, the present study compares the effect of two balanced anaesthetic procedures on respiration and circulation. In group 1 (midazolam/fentanyl/N2-O-O2/vecuronium), anaesthesia was induced with the usual bolus technique and maintained. In group 2 (midazolam/ketamine/N2-O-O2/vecuronium), a bolus was initially administered, while maintenance of the anaesthesia was achieved using a midazolam/ketamine perfusor (1 : 10). The blood pressure was recorded on an oscillograph, the heart rate by means of ECG and pulse oximetry. Post-operative determination of oxygen saturation was carried out using pulse oximetry. The blood pressure and heart rate curves in group 1 revealed stable intra-operative and Post-operative figures as compared with the initial values, with mild hypotension and decrease in pulse rate. In this group, hypoxic episodes may be expected during the recovery phase. In group 2, in part appreciable increases in blood Pressure and heart rate were observed in the induction phase. As compared with the initial values, blood pressure and heart rates were elevated during the intra-operative phase. Midazolam/ketamine anaesthesia revealed clear advantages in the post-operative phase, in the sense of more natural spontaneous respiration in comparison with midazolam/fentanyl anaesthesia.