In the present study, contributions of left-right reciprocal coupling mediated by commissural interneurons in spinal locomotor networks to rhythmogenesis were examined in larval lamprey that had longitudinal midline lesions in the rostral spinal cord (830% body length; BL, relative distance from the head) or caudal spinal cord (3050% BL). Motor activity was initiated from brain locomotor command systems in whole animals or in vitro brain/spinal cord preparations. Following midline lesions in the caudal spinal cord in whole animals and in vitro preparations, left-right alternating burst activity could be initiated in rostral and usually caudal regions of spinal motor networks. In in vitro preparations, blocking synaptic transmission in the rostral cord abolished burst activity in caudal hemi-spinal cords. Following midline lesions in the rostral spinal cord in whole animals, left-right alternating muscle burst activity was present in the caudal and sometimes the rostral body. Following spinal cord transections at 30% BL, rhythmic burst activity usually was no longer generated by rostral hemi-spinal cords. For in vitro preparations, very slow burst activity was sometimes present in isolated right and left rostral hemi-spinal cords, but the rhythmicity for this activity appeared to originate from the brain and the parameters of the activity were significantly different than those for normal locomotor activity. In summary, in larval lamprey under the present experimental conditions, left and right hemi-spinal cords did not generate rhythmic locomotor activity in response to descending inputs from the brain, suggesting that left-right reciprocal coupling contributes to both phase control and rhythmogenesis.