Total functional deadspace (including apparatus deadspace) was measured in anaesthetized patients while breathing from masks and after tracheal intubation. The mean VD/VT ratio was 0.18 larger during mask breathing and the mean value for the total functional deadspace was 82 ml greater than in the intubated patients. The results showed an increase in the VD/VT ratio during the inhalation of higher concentrations of halothane: the difference closely approached but did not attain the 95 per cent confidence limit. Values for VD/VT ratio and VD were uninfluenced by the duration of anaesthesia up to 120 minutes.