The aim of this study was to measure, in 11 patients with healthy lungs, active inspiratory impedance during anaesthesia. In addition, we recorded changes in inspiratory occlusion pressure at 100 ms (P0.1) and ventilatory pattern while awake and during anaesthesia with a mean inspiratory fraction (FI) of 0.017 halothane in O2. The total active inspiratory resistance and elastance values were 5.4 +/- 3.3 hPa.l.1.s and 29.9 +/- 6.2 hPa.l.1, respectively. P0.1 and the ratio between P0.1 and mean inspiratory flow (P0.1/(VT/TI)) increased 124% (p less than 0.001) and 68% (p less than 0.001), respectively, during anaesthesia. Respiratory frequency rose significantly from 12.2 +/- 1.5 (mean +/- SD) to 24.6 +/- 4.6 cycles.min-1, while tidal volume and inspiratory duty cycle lowered significantly from 0.599 +/- 0.195 l and 0.44 +/- 0.04 to 0.372 +/- 0.088 l (p less than 0.001) and 0.40 +/- 0.04 (p less than 0.05), respectively. Minute ventilation (VE) and VT/TI did not change significantly. During halothane anaesthesia with an FI:0.017, the increase in neuromuscular respiratory output appears to compensate for the increased mechanical load, thus resulting in maintenance of VE at levels similar to those of an awake state.