Modulation of synaptic potentials produced by electrical stimulation of low-threshold muscle afferents in lumbar alpha-motoneurons innervating knee and ankle muscles was studied by intracellular recording during "fictive locomotion" induced by stimulating the mesencephalic locomotor region (MLR) in paralyzed, mesencephalic cats. Averaging postsynaptic potentials in different phases of the fictive step cycle indicated that relatively little amplitude modulation occurred. In nearly half of the 38 motoneurons analyzed, there was a statistically significant tendency for excitatory postsynaptic potentials (EPSPs) to increase in amplitude during the depolarized phase of the oscillation in the membrane potential produced during fictive locomotion (locomotor-drive potential). In 8% the EPSPs decreased under the same conditions, while the rest displayed a constant amplitude during all phases of the fictive step cycle. Only three cells showed a distinct second peak in the EPSP at a latency consistent with transmission in a di- or trisynaptic pathway. Late inhibitory postsynaptic potentials (IPSPs) were also rarely observed. Thus oligosynaptic pathways from muscle afferents to the motoneuron groups studied are not prominent during the locomotor cycle in this preparation. We suggest that the marked modulation of monosynaptic reflex amplitude observed in mesencephalic cats (1) arises mainly from the effects of the locomotor-drive potential in bringing the cells closer to threshold during some phases of locomotion. Specific modulation during fictive locomotion of transmission in pathways from muscle afferents, which has been demonstrated for cutaneous pathways (28), was not observed. The implications of these results for the control of locomotion are discussed briefly.