The effect of thiopentone on the middle latency auditory evoked potentials was investigated in 12 patients during induction of anaesthesia with thiopentone 5 mg.kg-1. 100% oxygen was administered throughout the induction, and when the patient moved purposefully a further bolus (2 mg.kg-1) was given, and anaesthesia continued in the normal way. The middle latency auditory evoked potentials were elicited before and during the induction. Binaural clicks (70 dB above normal hearing threshold) were presented at a rate of 9.3 per s. Averages of 1000 responses were analysed off line, and a fast Fourier transformation of the middle latency auditory evoked potentials were used to calculate the power spectrum of the averages. Awake, the patients had large peak to peak amplitudes and a normal waveform. The power spectra showed a high energy between 30-40 Hz. After induction the latencies of waves Na, Pa, Nb and PI and the amplitudes of the waves Na/Pa, Pa/Nb, and Nb/PI were decreased or completely attenuated. Both effects lasted for 4 min. When movement occurred (after 4–6 min), the values returned to awake. The second bolus repeated the changes.