Studies on the circulatory effects of hyperventilation have been carried out in eight cases following Caesarean section. These indicate that hyperventilation with high inflationary pressures can cause large falls in cardiac output. In a further twenty-two cases undergoing Caesarean section measurements were made of blood gases before anaesthesia and at the time of delivery. The changes in PaCO2 and pH occurring during anaesthesia have been correlated with the umbilical vein pH at the time of birth. Foetal acid-base status moved in the same direction as that of the mother. The results have been discussed in relation to the possible harmful effects on the foetus of hyperventilation during anaesthesia for operative obstetrics.