In the mammalian spinal cord, Renshaw cells (RCs) are excited by axon collaterals of motoneurons (MNs), and in turn, provide recurrent inhibition of MNs. They are considered an important element in controlling the motor output. However, how RCs are modulated by spinal circuits during motor behaviors remains unclear. In this study, the physiological nature of inhibitory synaptic inputs to RCs in the lumbar segment during spontaneous motoneuronal activity was examined in the isolated spinal cord taken from glutamate decarboxylase 67-green fluorescent protein (GAD67-GFP) knock-in mouse neonates. Whole cell recordings of RCs in current-clamp mode showed that they receive phasic inhibition that could modulate the RC firing evoked by excitation of MNs. In voltage-clamp recording, we observed a barrage of spontaneous inhibitory postsynaptic currents (sIPSCs) mediated by glycine and/or GABA. These sIPSCs persisted in the presence of mecamylamine, a nicotinic receptor antagonist, indicating that excitation of other RCs by MN axon collaterals may not be essential for these inhibitory actions. Simultaneous recording of RC and the ventral root in the same segment showed that the RCs received inhibitory inputs when spontaneous MN firing occurred. Paired recordings of a RC and a MN showed that during the bursting activity in the ventral root, the magnitude of the RC sIPSCs and the magnitude of the excitatory inputs that MNs receive are highly correlated. These results indicate that RCs are modulated by inhibition that matches the MN excitation in timing and amplitude during motor behaviors.