Substance P has a powerful excitant action on spinal motoneurons of the newborn rat, and the action is specifically antagonized by Lioresal, which also readily blocks spinal reflexes involving primary afferent transmission. Substance P is highly concentrated in bovine and feline dorsal roots as well as in the dorsal part of the dorsal horn of cat spinal cord. There is evidence that substance P is synthesized in spinal ganglia, transported through the dorsal roots and stored in the axon terminals of primary afferent neurons. These and other results suggest that the undecapeptide substance P, or one of its shorter C-terminal analogues, functions as an excitatory transmitter of the primary afferent fibers.