The effects of stimulating the suprabulbar or bulbar reticular formation on reflex activity of the spinal cord have been studied by Lloyd*(1941, 1944), Magoun † (1944, 1950), Rhines and Magoun26(1946 ), Bach2(1950), Austin1(1952), Lindsley14(1952), and numerous others. The parameters of bulbar and suprabulbar facilitatory and inhibitory areas have been well worked out (Lindsley,141952; Lindsley, Schreiner, and Magoun,151949; Magoun and Rhines,231946; Niemer and Magoun,251947; Rhines and Magoun,261946), and the descending pathways from these centers are likewise known (Lloyd,161941; McCulloch, Graf, and Magoun,241946; Niemer and Magoun,251947; Thulin,281953). Less information is available, however, concerning the site and mode of action of the reticular formation on spinal reflex pathways. The purpose of the experiments to be reported here was that of studying the mode of the facilitatory action