[Leu5]-enkephalin is an opioid peptide that has been found to modulate nociception in the spinal cord. This opiate peptide functions as a modulator of neurotransmission. [Leu5]-enkephalin induces circular muscle contractions and reduces immunoreactive vasoactive intestinal peptide secretion. Some anatomical results suggest that neurons containing immunoreactive [Leu5]-enkephalin in the dorsal horn, some of which may receive input from primary afferents, modulate nociception by directly synapsing with cells of origin of the spinothalamic tract and also by interacting with primary afferent terminals.