The influence of different levels of carbon dioxide during general anaesthesia on postoperative recovery was studied. Sixty female patients were divided into two age groups. Thirty patients were over 60 years old and 30 patients were under 46 years old. Artificial ventilation with added carbon dioxide during general anaesthesia with thiopentone, nitrous oxide-oxygen, alcuronium and fentanyl was used. According to the arterial CO2 tension, patients were allocated to subgroups: hypercarbic, PaCO2 7.3 kPa, normocarbic, PaCO2 5.3 kPa and two different levels of hypocarbia: older patients PaCO2 3.7 kPa and younger patients PaCO2 2.9 kPa. As criteria for recovery, a battery of recovery tests and postoperative questionnaires were used. Regardless of age, patients subjected to hypercarbic ventilation scored better in the recovery tests than patients subjected to normo- or hypocarbia. Normocarbic ventilation also gave better results than hypocarbic ventilation. The level of hypocarbia used in the older patients and that used in the younger patients, though different, resulted in nearly the same deterioration of scoring in the recovery tests. This deterioration was seen in some patients up to 48 h postoperatively. No subjective differences were elicited from the questionnaires after various types of ventilation.