1. Primary afferent depolarization (PAD) can be evoked by sensory volleys, supraspinal commands, or the activity of spinal locomotor networks (locomotor-related PAD). In this study we investigated the effect of locomotor-related PAD and of sensory-evoked PAD on the monosynaptic transmission between the group IA muscle afferents and motoneurons in the lumbosacral spinal cord. 2. Six pairs of group IA afferents and motoneurons [4 tibialis anterior (TA), 1 medial gastrocnemius (MG), 1 lateral gastrocnemius-soleus (LGS)] were successfully recorded intracellularly during spontaneous fictive locomotion in the decerebrate cat. The membrane potentials of TA axons and motoneurons were maximally depolarized during the flexor phase of the locomotor cycle. In MG and LGS pairs, the maximum depolarization in IA axons occurred during the flexor phase and, in motoneurons, during the extensor phase. There were no antidromic discharges in the recorded axons. The effects of locomotor-related PAD on IA transmission were evaluated by comparing the unitary excitatory postsynaptic potentials (EPSPs) in the motoneuron evoked by the spontaneous orthodromic firing of the group IA axon during the flexor and extensor phases, respectively. In TA pairs, the maximum amplitude of unitary EPSPs occurred during the flexor phase when the motoneuron and the axon were maximally depolarized. In the MG and LGS pairs, the maximal amplitude of unitary EPSPs occurred during the extensor phase when the motoneuron was maximally depolarized and when the axon was the least depolarized. Overall, the amplitude of unitary EPSPs was clearly modulated during the fictive step cycle and always reached a maximum during the depolarized phase of the motoneuron, whether the group IA axon was maximally depolarized or not during that phase. 3. The effect of sensory-evoked PAD on synaptic transmission was also studied in nonlocomoting preparations. One TA pair was successfully recorded and PADs were evoked by the stimulation of a peripheral nerve. The amplitude of unitary EPSPs in the motoneuron was greatly depressed during the PADs. This result is a direct demonstration of the presynaptic inhibition associated with the sensory-evoked PAD in the monosynaptic reflex pathway of the cat. 4. We conclude from these results that the locomotor-related PAD did not contribute significantly to the modulation of transmission in the monosynaptic reflex pathway of the cat during fictive locomotion. On the other hand, the results confirmed that PAD evoked by sensory input decreases group IA afferent transmission efficiently most probably by presynaptic inhibition. The results suggest therefore that, during real locomotion, sensory feedback induced by the moving limbs or perturbations will evoke an important presynaptic inhibition of the release from group IA primary afferent terminals.