The decrease in arterial oxygen tension, during 60 sec of apnoea with exposure to room air, was studied in twelve healthy parturient women undergoing Caesarean section under endotracheal fluroxene-oxygen anaesthesia. Eight young gynaecological patients, anaesthetized in an identical manner, served as controls. The reduction in oxygen tension was significantly greater in the pregnant women than in the controls and was greatest in women in labour. The results confirm the presence of a markedly increased oxygen consumption at term and emphasize the importance in obstetric anaesthesia of preoxygenation before and of prompt reoxygenation following endotracheal intubation.