We have examined the regionalization and coordination of rhythm- generating networks in the isolated spinal cord of the chick embryo between embryonic days 9 and 13, by recording the pattern of rhythmic activity recorded from muscle nerves and ventral roots following a variety of lesions. We found that the capacity for rhythmic activity is distributed along the rostrocaudal axis of the cord but can be expressed in a single, isolated segment. Specializations within the lumbosacral cord were investigated by isolating particular regions and recording their motor output. The rostral part of the lumbosacral cord generates more cycles than the caudal part, and this difference becomes more pronounced with development. In the unlesioned cord, motoneuron activity is synchronized along the rostrocaudal axis. Lesion experiments revealed that the synchronization of motoneuron activity and the synaptic drive to caudal motoneurons is mediated in part by propriospinal pathways traveling in the ventrolateral white matter tracts and by synaptic interactions within the gray matter. The dorsal fiber tracts may also be involved but their effects appear to be weak. Lesions in dorsal-ventral and mediolateral planes were used to localize regions critical for rhythmogenesis and for the alternation of flexor and extensor motoneurons. Rhythmic activity with alternation persisted in spinal cords in which the dorsal and medial half had been removed. Severe medial or dorsal lesions, resulting in a thin strip of lateral or ventral gray matter, altered the phasing of motoneuron activity from alternating to synchronous without effects on cycle timing. These results suggest that the critical neural components for alternation are located close to and dorsomedial to the lateral motor column, and that the capacity for rhythmogenesis is distributed widely throughout the ventral gray matter and is not localized to specific nuclei.