Electrical microstimulation of specific areas of the brainstem in an in vitro preparation of the lamprey CNS elicits coordinated swimming motor activity recorded in spinal ventral roots. Swimming motor activity could be graded in intensity by variations in stimulus current. This 'brainstem locomotor region' extends from the mesencephalon, near the torus semicircularis, caudally through the rhombencephalon in parallel strips about 200-300 micron lateral to the midline.