Pulmonary gas distribution, functional residual capacity (FRC), closing capacity (CC), arterial oxygen tension (PaO2) and alveolar-arterial oxygen tension gradient (PAO2-PAO2) were measured in seven subjects before and after the induction of extradural analgesia for routine surgery. It was found that pulmonary gas distribution was within normal limits throughout the study, although there were two patients in whom airway closure occurred consistently within the tidal volume. In both cases this was associated with a low PaO2. CC and FRC were substantially unchanged by the induction of extradural analgesia. Changes in (PAO2-PaO2) and PaO2 were usually not large, and are apparently related to factors other than changes in lung geometry.