The flattened lamprey spinal cord contains stretch-sensitive edge cells located along the lateral margin, with dendritic processes sensing the lateral bending of the cord during each swim cycle. These intraspinal stretch receptor neurons provide movement-related sensory feedback input to the generator network for locomotion causing a powerful entrainment of the rhythm. In order to elucidate the synaptic effects of edge cells we have performed paired intracellular recordings and staining with Lucifer yellow. Monosynaptic connections that may explain entrainment were found to locomotor central pattern generator interneurons. Edge cells with an ipsilateral axon elicited excitatory postsynaptic potentials (EPSPs) in ipsilateral interneurons. In addition, such edge cells evoked kainate/quisqualate receptor mediated EPSPs in ipsilateral motoneurons. This pathway mediates an intraspinal stretch reflex analogous to the muscle spindle mediated stretch reflex of mammals. Edge cells with a contralateral axon produced monosynaptic glycinergic IPSPs in contralateral neurons, including contralateral edge cells.