The cumulative dose effect of ketamine hydrochloride (KH) on transcranial magnetic-induced motor evoked potentials (MEPs) was examined in monkeys. Electromyographic (EMG) responses were recorded from the contralateral abductor pollicis brevis (APB) and abductor hallucis (AH) muscles. MEP brain stimulation threshold, latency, and amplitude values were studied. After obtaining baseline recordings, increments of KH (5 mg/kg every 15-20 min) were given i.v. (50 mg/kg total dose). MEPs were repeatedly recorded following KH injections. No loss of potentials was encountered in any animal. However, KH induced significant MEP latency delay in doses >/=35-40 mg/kg and amplitude depression in doses >/=15-20 mg/kg (p < 0.01). Under various KH doses, the amplitude depression ranged from 13.6 to 45.5% for APB and 57.3 to 82% for AH compared to the control values. The MEP latency prolongation ranged from 3.5 to 18% for APB and 4.2 to 13.1% for AH. The stimulation threshold rise ranged from 6.7 to 14.7% for APB and 7 to 17.9% for AH. Statistical correlation was closest between cumulative KH doses and MEP latency prolongation. We conclude that, in the primate model, reliable MEP recording is feasible under deep KH anesthesia. However, awareness of drug-induced response alterations is essential during interpretation of intraoperative MEP changes. Further investigation is warranted regarding the specific dose effect in humans and safety of magnetic stimulation.