Changes in arterial oxygen saturation during induction of anaesthesia and intubation were studied using the pulse oximeter. Seventy-five young ASA I patients undergoing elective uncomplicated surgery were divided equally into three groups. The patients were preoxygenated with 100% oxygen, 50% oxygen: 50% nitrous oxide or 30% oxygen: 70% nitrous oxide for 1 min. All were then induced with thiopentone, paralysed with suxamethonium and orally intubated. Arterial oxygen saturations were continuously recorded by a separate investigator. All groups showed similar arterial desaturation during suxamethonium-induced apnoea and intubation, but the degree of desaturation was not clinically significant and no patient showed clinical signs of hypoxaemia. Preoxygenation with mixtures of oxygen and nitrous oxide can hasten the build-up of alveolar nitrous oxide concentration and help to smooth induction without compromising oxygenation of patients.