To determine the effects of induction agent and obesity on the change in functional residual capacity caused by induction of anaesthesia, we measured the change in end-expiratory respiratory volume (EERV) during induction of anaesthesia with either thiopentone or propofol. Female patients breathed via a face mask from a closed circle system connected to a water spirometer. The spirometer tracing was used to measure the decrease in EERV. The median decrease was 200 (95% confidence limits 90-280) ml after thiopentone and 285 (200-425) ml after propofol. The decrease in each group was significant (P < 0.01) but there was no difference between the two groups. The change in EERV, expressed as a proportion of the predicted functional residual capacity, was not related to obesity (estimated as the ratio of actual to predicted body weight).