Fifty patients were randomly allocated to receive either a preinduction inhalation with nitrous oxide (50%) in oxygen or fentanyl with preoxygenation, before induction of anaesthesia with propofol. Both groups of patients showed a significant rise in arterial oxygen saturation prior to propofol induction which established similar depths of anaesthesia, determined by the acceptability of the laryngeal mask placement. In the fentanyl group there was a significant period of apnoea after induction, with 40% of the patients being apnoeic for 120s or more and requiring assisted ventilation. Reduction in arterial blood pressure was also more rapid in the fentanyl group compared with the nitrous oxide group. Preinduction inhalation of nitrous oxide (50%) in oxygen appears to be an effective and acceptable method of preoxygenating the patient and augmenting the propofol induction of anaesthesia.