Cortical and spinal root (C(8)-T(1)) magnetic stimulation evoked single motor unit potentials in the abductor pollicis brevis and abductor digiti minimi muscles in a patient with chronic motor neuron disease. This patient was unique in that there were few surviving motoneurons in these muscles making it possible to record single motor units. Central motor conduction time was within normal limits. Cortical mapping of motor evoked potentials (MEPs) was carried out using a twin magnetic coil stimulating over the motor cortex at intervals of 1 cm along the coronal axis and 1-2 cm along the sagittal axis. As the site of cortical stimulation was moved from the centre, the latency of the MEPs increased by 0.7-0.8 ms suggesting one synaptic delay. This study provides further data that magnetic stimulation of the human cortex indirectly activates pyramidal cells via interneurons.