The effects of three anesthetic induction agents on somatosensory evoked potentials (SEP) were assessed in unpremedicated patients who were without neurologic abnormality of the upper extremities. SEP was assessed by stimulation of the nondominant median nerve and responses were recorded over Erbs point (N10), second cervical vertebra (N14), and the contralateral cortex (P15, N20, P23 latencies, and P15-N20 and N20-P23 amplitudes). Nine patients received thipental (4 mg/kg, iv bolus), nine patients received fentanyl (25 micrograms/kg, iv bolus), and nine patients received etomidate (0.4 mg/kg, iv bolus). SEP was assessed before and after drug administration at motor threshold stimulus intensity. Thiopental increased the latency of N10, N14, and N20. The amplitudes of N10-, N14-, and scalp-recorded waves were not altered by thiopental. Fentanyl increased N20 and P23 latency and decreased the amplitude of P15-N20. Etomidate increased latency of N20 and P23 without alteration of latencies of N10 or N14 and increased the amplitude of P15-N20 and N20-P23, while the amplitude of N10 was unchanged and the amplitude of N14 was decreased. It is concluded that thiopental or fentanyl causes only modest alterations in early waves of upper extremity SEP, whereas etomidate increases the amplitude of scalp-recorded waves. The effect of etomidate on SEP may make diagnosis of neurologic injury more difficult because of the changing waveform.