Tidal volume, respiratory timing and occlusion pressure were studied in spontaneously breathing patients during isoflurane anaesthesia and in the presence of surgery. In one group of patients, end-tidal concentrations of 1.5% and 2.5% isoflurane in oxygen were used and in the other group equivalent depths of anaesthesia were obtained with 0.75% and 1.75% end-tidal isoflurane plus 67% nitrous oxide in oxygen. During the deeper anaesthesia, the duration of the respiratory cycle decreased significantly in both groups. In the group breathing nitrous oxide, the greater isoflurane concentration caused a significant reduction in ventilation. Airway occlusion significantly decreased the duration of the inspiratory period in both groups, indicating that the presence of nitrous oxide was not necessary for this effect.