Seventeen patients undergoing routine laparoscopy, using carbon dioxide as the insufflating gas, were anaesthetized by a technique using halothane in oxygen and permitting spontaneous ventilation. The ventilatory variables and electrocardiographic changes were monitored continuously or at 2-3 min intervals. Little change in the ventilatory variables was noted when die abdomen was distended but they altered considerably after the gynaecological procedure had been completed. No cardiac arrhythmias occurred after carbon dioxide had been introduced into the peritoneal cavity. The possible causes of the cardiac irregularities seen when carbon dioxide insufflating gas is used for laparoscopy when spontaneous ventilation with halothane is used are discussed.