We have investigated changes in respiratory sinus arrhythmia (RSA) and compared these with clinical signs of anaesthesia in children. Children aged 3-10 yr were anaesthetized by gaseous induction with halothane and nitrous oxide. Multiple heart rate variability (HRV) spectra were obtained by power spectral analysis of continuous epochs of time from before introduction of halothane (baseline) until the pupils were central and fixed (stage 3). Measurement of RSA was performed by integration of the area under the spectral curve within the range of the respiratory frequency +/- 0.15 Hz. In all patients RSA decreased continuously during induction unless stimulation occurred with insertion of an airway. Values of RSA were compared at three times: baseline, loss of pharyngeal tone and stage 3. The decrease in RSA from baseline to loss of pharyngeal tone and from loss of pharyngeal tone to stage 3 was significant (P = 0.003 and P = 0.018, respectively). These results show that RSA can be related to the clinical signs of anaesthesia and has potential as a measure of depth of anaesthesia in children.