The activity of motoneurons during lateral turns was studied in a lower vertebrate, the lamprey, to investigate how a supraspinal command for the change of direction during locomotion is transmitted from the brain stem and integrated with the activity of the spinal locomotor pattern generator. Three types of experiments were performed. 1) The muscular activity during lateral turns in freely swimming adult lampreys was recorded by electromyography (EMG). It was characterized by increased cycle duration and increased duration, intensity, and cycle proportion of the bursts on the side toward which the animal turns. 2) Electrical stimulation of the skin on one side of the head in a head-spinal cord preparation of the lamprey during fictive locomotion elicited asymmetric ventral root burst activity with similar characteristics as observed in the EMG of intact lampreys during lateral turns. The cycle duration and ventral root burst intensity, duration, and cycle proportion on the side of the spinal cord contralateral to the stimulus were increased; hence a fictive lateral turn away from the stimulus could be produced. The fictive turn propagated caudally with decreasing amplitude. The increase in burst duration during the turn correlated well with the increase in cycle duration, while changes in contralateral burst intensity and burst duration did not co-vary. Turning responses varied depending on the timing (phase) of the skin stimulation: stimuli in the first two-thirds of a cycle evoked a turn in the same cycle, whereas stimuli in the last third gave a turn in the following cycle. The largest turns were evoked by stimuli in the first third of a cycle. 3) Fictive turns were abolished after transection of the trigeminal nerve or a rhombencephalic midline split, but not in a rhombencephalic preparation with transected cerebellar commissure. High spinal hemisection was sufficient to block turning toward the lesioned side, while turns toward the intact side remained. Taken together these findings suggest that the reticulospinal turn command is essentially unilateral and generated in the rhombencephalon.