During artificial ventilation of anaesthetised patients the respiratory minute volumes were estimated after end tidal CO2-values (eeCO2) and correlated to arterial blood gases. In men the mean respiratory minute volume of 130 ml/kg body weight (b.w.) was significantly above the 113 ml/kg b.w. of women. 9.7-year-old boys and 16-year-old men had higher minute volumes (178 ml/kg b.w. and 148 ml/kg b.w. respectively) in comparison to men of 23 years of age. In contrast, no significant difference was seen in the older age groups. Overweight men had a significant lower respiratory minute volume (114 ml/kg b.w.), as compared to normal body weight (128 ml/kg b.w.), but overweight women had no significantly different minute volumes compared to normal body weight. During the course of anaesthesia for vaginal hysterectomy the respiratory minute volume had to be reduced in the first and second hour as compared to controls before the start of operation, in order to avoid excessive hyperventilation. EeCO2-values of 3.8 to 4.2 per cent by volume stand for a very marked hyperventilation in the pulmonary healthy patient. The arterial alveolar difference of the CO2 pressure (aADCO2) was in the normal range between 1 and 5 mmHg. The end tidal CO2 estimation is a noninvasive and suitable method to exactly meet the ventilatory needs of the anaesthetised patient.