A formula is derived for maintaining normocapnia during controlled ventilation using a circle system without carbon dioxide absorption. In a series of 70 patients, unselected in terms of age, sex, obesity, ASA status, body position during operation, type of anaesthetic administered or type of circle system used, it was found that a total fresh gas flow of 50 ml/kg body weight/min and a minute ventilation of 120 to 150 ml/kg body weight at a rate of 10 to 12/min achieved normocapnia. For moderate hypocapnia a total fresh gas flow of 60 ml/kg body weight/min and a minute ventilation of 120 ml/kg at a rate of 10-12/min is suggested.