Magnetic stimulation of the brain and cervical and lumbar spinal roots was performed on 48 healthy subjects in order to investigate the effects of sex, height and age on motor evoked potentials (MEPs). The compound muscle action potentials were recorded from the abductor pollicis brevis and abductor hallucis muscles. The central motor conduction time (CMCT) was measured between the cerebral cortex and the cervical root and also between the cerebral cortex and the lumbar root. A multiple regression analysis was used to determine which of the physical variables, namely sex, height and age, were significant. A significant gender difference was observed in the MEP latencies and CMCT of the leg, but not in those of the hand. Both height and age had a significant effect on the leg MEP latencies with a lesser effect on the hand MEP latencies. The leg CMCT was also significantly influenced by height and age, while the hand CMCT was not. These results thus suggest that physical variables are very important in defining normal MEPs, especially in the lower limbs. Therefore, when we assess motor function in patients with neurological disorders, both the patients and control groups should be matched for sex, height and age distribution.