An isolated lamprey spinal cord generates rhythmic ventral root bursting that closely resembles swimming activity: ventral roots on opposite sides of the spinal cord burst in alternation, and rostral ventral roots lead caudal ventral roots. This rhythmic activity is induced by superfusion of the spinal cord with an excitatory amino acid and is called 'fictive' swimming. Three main types of spinal interneurons that are active during fictive swimming have been characterized: small excitatory interneurons with ipsilateral axons, large inhibitory interneurons with ipsilateral descending axons (lateral interneurons), and inhibitory commissural interneurons. The synaptic connectivities of these inter-neurons can account for the pattern of motoneuron excitation and inhibition occurring during fictive swimming, and it has been proposed that the synaptic interactions of these spinal neurons form the unit segmental oscillators of the swim network. Computer modeling has confirmed that this network can generate rhythmic activity resembling fictive swimming. The core of the model is the reciprocal inhibition between commissural interneurons on opposite sides of the cord. Evidence that the commissural interneurons are essential to rhythm generation comes from lesion studies in which the spinal cord was split down the midline and also from photo-ablation studies in which commissural interneurons were inactivated by illumination after retrograde labeling with a photo-toxic tracer. In both types of experiments, rhythmic activity of fictive swimming can be abolished, supporting the view that the commissural interneurons are necessary for rhythmogenesis.