We examined the effects of spontaneous or evoked episodes of rhythmic activity on synaptic transmission in several spinal pathways of embryonic day 9-12 chick embryos. We compared the amplitude of synaptic potentials evoked by stimulation of the ventrolateral funiculus (VLF), the dorsal or ventral roots, before and after episodes of activity. With the exception of the short-latency responses evoked by dorsal root stimulation, the potentials were briefly potentiated and then reduced for several minutes after an episode of rhythmic activity. Their amplitude progressively recovered in the interval between successive episodes. The lack of post-episode depression in the short-latency component of the dorsal root evoked responses is probably attributable to the absence of firing in cut muscle afferents during an episode of activity. The post-episode depression of VLF-evoked potentials was mimicked by prolonged stimulation of the VLF, subthreshold for an episode of activity. By contrast, antidromically induced motoneuron firing and the accompanying calcium entry did not depress VLF-evoked potentials recorded from the stimulated ventral root. In addition, post-episode depression of VLF-evoked synaptic currents was observed in voltage-clamped spinal neurons. Collectively, these findings suggest that somatic postsynaptic activity and calcium entry are not required for the depression. We propose instead that the mechanism may involve a form of long-lasting activity-induced synaptic depression, possibly a combination of transmitter depletion and ligand-induced changes in the postsynaptic current accompanying transmitter release. This activity-dependent depression appears to be an important mechanism underlying the occurrence of spontaneous activity in developing spinal networks.