In spite of many reports of arrest reaction in animals, there are very few reports in man. During a therapeutic stereotactic operation we observed peculiar phenomena caused by electrical stimulation to the deep structure of the cerebrum. This phenomenon is quite similar to that reported by Van Buren, but there are a few differences between them. Arrest reaction was observed in 14 of 23 cases, 17 of 28 tracks. Stimuli which caused the arrest reaction were 60–100 Hz square waves of 1 msec duration and 5–15 V. The electrical stimulation caused an interruption of counting and other motor actions, which could be resumed following release of stimuli. Psychic confusion or memory disturbances were not observed with the exception of a few cases. The arrest reaction that we observed is thought to be due to a direct effect on the head of the caudate nucleus, not due to secondary effects on the internal capsule and the motor fiber in the vicinity of the caudate nucleus. However, the possibility that the current spread to the motor fiber cannot be definitely ruled out.