Total respiratory compliance was measured before and after delivery of the baby during Caesarean section performed under relaxant anaesthesia. Because of the smaller size of the uterus after delivery the abdominal contents no longer press so hard on the diaphragam. At the same time the pulmonary blood volume probably becomes less. The result is a mean increase in compliance of about 20 per cent. When using a pressure-cycled ventilator this increase results in the delivery of a greater tidal volume. This may have unforeseen consequences.