The presynaptic inhibitory action of Group I afferent fibers has been further investigated by the standard techniques: the intracellular and extracellular recording of depolarization of the primary afferent fibers; the testing of excitability changes of the primary afferent fibers; the recording of dorsal root potentials and potentials from the cord dorsum. Group la afferent fibers are depolarized by synaptic action of both Group la and Ib afferent volleys from flexor muscles. Three properties demonstrated for the central pathways subserving presynaptic inhibitory action indicate their polysynaptic character: temporal facilitation, spatial facilitation, and posttetanic potentiation. During prolonged repetititve stimulation there is firstly a building up of primary afferent depolarization by the first few impulses, then a progressive decline to a plateau that is reached fa half to one second, after which it is well maintained over many seconds.