The effects of opioids on sacrocaudal afferent (SCA) pathways and the pattern generating circuitry of the thoracolumbar and sacrocaudal segments of the spinal cord was studied in isolated spinal cord and brainstem-spinal cord preparations of the neonatal rat. The locomotor and tail moving rhythm produced by activation of nociceptive and non-nociceptive sacrocaudal afferents was completely blocked by specific application of the µ-opioid receptor agonist DAMGO to the sacrocaudal, but not the thoracolumbar segments of the spinal cord. The rhythmic activity could be restored after addition of the opioid receptor antagonist naloxone to the experimental chamber. The opioid block of the SCA induced rhythm is not due to impaired rhythmogenic capacity of the spinal cord, because a robust rhythmic activity could be initiated in the thoracolumbar and sacrocaudal segments in the presence of DAMGO, either by stimulation of the ventromedial medulla or by bath application of NMDA/5HT. We suggest that the opioid block of the SCA-induced rhythm involves suppression of synaptic transmission through sacrocaudal interneurons interposed between SCA and the pattern generating circuitry. The expression of µ opioid receptors in several groups of dorsal, intermediate and ventral horn interneurons in the sacrocaudal segments of the cord, documented in this study, provides an anatomical basis for this suggestion.