Locomotor modulation of disynaptic EPSPs from the mesencephalic locomotor region in cat motoneurons. J. Neurophysiol. 80: 3284-3296, 1998. When low-frequency tetanization of the mesencephalic locomotor region (MLR) produce fictive locomotion in unanesthetized, decerebrate cats, each MLR stimulus produces a distinctive cord dorsum potential (CDP) and oligosynaptic excitatory postsynaptic potentials (EPSPs) in many lumbosacral motoneurons. The average segmental latency from the initial CDP wave [mean delay from stimulus: 4.3 +/- 0.9 (SD) ms] to the onset of detectable MLR EPSPs was 1.6 +/- 0.4 ms, suggesting a disynaptic segmental connection. In gastrocnemius/soleus, flexor hallucis longus, flexor digitorum longus, tibialis anterior, and posterior biceps-semitendinosus motoneurons (35/38 cells), MLR EPSPs either appeared or were enhanced during the phase of fictive stepping in which the target motoneurons were depolarized and the motor pool was active (the phase), with parallel changes between EPSP amplitudes and membrane depolarization. In contrast, MLR stimulation produced small (1/10) or no EPSPs in extensor digitorum longus (EDL) motoneurons, with no phase enhancement (4/10) or oligosynaptic inhibitory postsynaptic potentials during the phase (5/10). Eight of 10 flexor digitorum longus (FDL) cells exhibited membrane depolarization in the early flexion phase of fictive stepping, and five of these showed parallel enhancement of disynaptic MLR EPSPs during early flexion. Three cases were studied when the FDL motor pool exhibited exclusively extensor phase firing. In these cases, the disynaptic MLR EPSPs were enhanced only during the extensor phase, accompanied by membrane depolarizations. We conclude that the last-order interneurons that produce disynaptic MLR EPSPs may well participate in producing the depolarizing locomotor drive potentials (LDPs) found in hindlimb motoneurons during fictive locomotion. However, the absence of linkage between MLR EPSP enhancement and LDP depolarizations in EDL motoneurons suggests that other types of excitatory interneurons also must be involved at least in some motor pools. We compared these patterns with the modulation of disynaptic EPSPs produced in FDL cells by stimulation of the medial longitudinal fasciculus (MLF). In all seven FDL motoneurons tested, disynaptic MLF EPSPs appeared only during the extension phase, regardless of when the FDL motoneurons were active. The fact that the modulation patterns of MLR and MLF disynaptic EPSPs is different in FDL motoneurons indicates that the two pathways do not converge on common last-order interneurons to that motor pool.