This study compares some characteristics of the disynaptic excitatory pathways from the lateral vestibular nucleus (LVN) and medial longitudinal fasciculus (MLF) to lumbosacral alpha-motoneurons in the decerebrate cat. We used the spatial facilitation technique to test whether disynaptic LVN and MLF excitatory postsynaptic potentials (EPSPs) are produced by common last-order interneurons in the lumbosacral segments of the spinal cord. Of 77 motoneurons examined, 26 exhibited disynaptic EPSPs from both supraspinal sources. No spatial facilitation was found between LVN and MLF EPSPs in 21 of 24 cells that were adequately tested. In 3 of 23 cells (all flexor motoneurons), some spatial facilitation was found in some but not all trials. These observations suggest that stimulation of the LVN and MLF produces disynaptic EPSPs in motoneurons through largely separate populations of last-order interneurons. Disynaptic MLF and LVN EPSPs showed parallel patterns of modulation during fictive locomotion. Maximal disynaptic EPSP amplitudes occurred during the phase of the step cycle when the recorded motoneuron, whether flexor or extensor, exhibited depolarizing locomotor drive potentials and the corresponding muscle nerve was active. These observations, taken together, suggest that disynaptic LVN and MLF EPSPs are produced in motoneurons by at least four separate populations of segmental last-order excitatory interneurons, with separate populations projecting to flexor versus extensor cells. The results also suggest that the modulation of the disynaptic EPSPs during fictive locomotion is mainly due to premotoneuronal convergence of input from the respective descending systems and from the segmental central pattern generator for locomotion onto common interneurons.