The effects of single electrical pulse stimulation of the contralateral precentral gyrus on lumbosacral monosynaptic reflexes have been studied in unanesthetized monkeys in which the pontomesencephalic junction was destroyed except for the pyramidal tracts. These preparations have been designated pyramidal monkeys. The precentral gyrus has been shown to have both facilitatory and inhibitory effects on motoneuron populations of the lumbosacral spinal segments. This cortically evoked facilitation and inhibition could not be separated at the cortical level. The pathway conducting the early cortical effects on monosynaptic reflexes appeared to be relatively simple and perhaps direct. Single pulse stimulation of the bulbar pyramid duplicated the results obtained by precentral gyrus stimulation whereas single pulse stimulation of non-pyramidal brainstem regions caudal to the lesion failed to elicit similar effects on the test mono-synaptic reflex. The observed pattern of cortical and pyramidal facilita-tion and inhibition was the same for monosynaptic reflexes initiated either by dorsal root stimulation or by stimulation of peripheral hind-limb nerves. Test monosynaptic reflexes elicited either by stimulation of flexor muscle nerves or by stimulation of extensor muscle nerves were similarly influenced by these suprasegmental projections. The basic pattern of cortical facilitation and inhibition of monosynaptic reflexes has been shown to be constant and not altered by tensions applied to synergistic and/or antagonistic muscles related to the muscle nerve used to elicit the reflex.