Substance P is present in numerous organs (salivary glands, trachea, pancreas, kidneys, bladder and prostate) and in various parts of the central and peripheral nervous system, notably substantia nigra, hypothalamus, pineal body and dorsal horn of the spinal cord. It is a potent stimulant of salivary secretion and intestinal motility, a vasodilator in muscles and fatty tissues and an inhibitor of insulin release. Its main role, however, lies in the transmission of pain, where it seems to act as neuromodulator. Released when nociceptive fibers are activated at the same time as the fast-acting neuromediator, it enhances and prolongs the effects of the latter. Opiates inhibit its release. In addition, substance P is present in the excitatory neurones of the corpus striatum-substantia nigra pathway, which also has GABA-containing inhibitory neurones, and this pathway is known to modulate the dopaminergic nigra-striatum pathway. The global function of substance P, therefore, seems to keep the central nervous system in a state of alert through activation of the cerebral cortex and assistance in the transmission of pain.