Synthetic substance P (SP) raises glycine levels both in the brain and in the spinal cord. The synthetic polypeptide, as well as the single amino acid, abolishes motor hyperactivity induced by β,β′-iminodipropionitrile in mice. The amino acid is effective event at very low intrinsic glycine levels, a condition where the polypeptide fails to act. It is suggested that SP action against exaggerated motories is based on its increasing of spinal glycine which, in turn, inhibits the motor neurones.