The actions of two new mammalian tachykinins, neurokinin alpha and neurokinin beta, were examined using the isolated spinal cords of newborn rats. Depolarizing responses of spinal motoneurons were recorded extracellularly from the lumbar ventral root during application of neurokinin alpha or neurokinin beta at concentrations ranging from 3 X 10(-8) M to 10(-6) M. The potencies of various tachykinins in depolarizing the motoneurons showed the following order: physalaemin greater than neurokinin beta divided by kassinin divided by substance P greater than neurokinin alpha. When the synaptic transmission in the spinal cord was blocked by tetrodotoxin, the depolarizing action of neurokinin alpha and neurokinin beta was markedly reduced but not completely abolished. The depolarizing action of neurokinin alpha and neurokinin beta was depressed by a substance P antagonist, [D-Arg1, D-Pro2, D-Trp7,9, Leu11]SP. The possibility that neurokinin alpha and neurokinin beta act as neurotransmitters in the mammalian spinal cord is discussed.