Purity >95%. [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.