This study examined the variation in cardiac inter-beat interval during induction of anaesthesia with propofol and subsequent inhalation anaesthesia with nitrous oxide and isoflurane. In comparison with preoperative control values, heart rate variability was reduced by anaesthesia and there was a complex, but consistent, pattern of R-R interval change during induction. Immediately after propofol 2 mg kg-1, high frequency heart rate oscillations were replaced by low frequency oscillations (0.05 Hz). Subsequently, with decreasing propofol and deepening nitrous oxide-isoflurane anaesthesia, high frequency components returned towards, although not reaching, control values; low and, to a greater extent, mid range (0.1-Hz) frequency components remained reduced. We postulate that these alterations are consistent with an immediate but transient post-induction ventilatory depression, subsequent decrease in efferent sympathetic activity and reduction in baroreflex responsiveness. R-R interval analysis is suggested to be a useful tool in the evaluation of autonomic function during anaesthesia.