The development of hindlimb motor activity was studied in an isolated preparation of the chick spinal cord. The motor output from lumbosacral segments was characterized by recording the pattern of ventral root and muscle nerve discharge in 6-14-d-old embryos. In addition, the synaptic drive underlying motoneuron activity was monitored electrotonically from the ventral roots. Spontaneous motor activity consisted of recurring episodes of cyclical motoneuron discharge. During development, both the number of cycles in each episode and the intensity of discharge in each cycle progressively increased. Monophasic, positive ventral root potentials accompanied each cycle of motoneuron discharge. Prior to the innervation of hindlimb muscles at stage 26, ventral root discharge was barely detectable despite the presence of large ventral root potentials. Following hindlimb muscle innervation, each cycle of activity was initiated by a brief, intense discharge that coincided with the rising phase of the ventral root potential. In embryos older than stage 30, the initial discharge was followed, after a delay, by a more prolonged discharge. The duration of ventral root potentials was shortest in the stage 26 embryos, but was similar in embryos at stage 29 and older. The developmental changes in the coordination of antagonist activity were documented by recording the pattern of discharge in sartorius (flexor) and caudilioflexorius (extensor) muscle nerves between stage 30 and stage 36. At stage 30 both sets of motoneurons were coactivated during the brief discharge that initiated each cycle. By stage 31 a second discharge occurred in each cycle. The second discharge was delayed in flexor, but not in extensor, motoneurons, which led to an alternating pattern of activity.(ABSTRACT TRUNCATED AT 250 WORDS)