In decorticate, unanaesthetized and curarized rabbit preparations displaying spontaneous fictive locomotor sequences, the firing pattern of neurones was recorded extracellularly in the L6-S1 spinal cord. These neurones, located in the intermediate part of the cord, were not invaded by antidromic stimulation of the hindlimb muscle nerves and thus were considered as interneurones (or propriospinal or tract cells ascending to the brain). When compared to the output from the ipsilateral muscle nerves, these neurones were classified as flexor (F INs) or extensor (E INs) according to the phase of the locomotor cycle when they displayed their maximal firing rate. Among 69 F INs, 33 maintained tonic firing during the periods between episodes of locomotor activity. Their maximal firing rate was in phase with the flexor efferent bursts of the locomotor sequence; during the extensor phase, they maintained an instantaneous frequency (i.f.) that was clearly above the resting i.f. Of these neurones, six became completely silent during the initial flexorextensor coactivation that opened the sequence (F1 neurones) whereas the 27 others increased their firing rate at that time (F2 neurones). The other neurones (36 F3) were silent between the locomotor episodes. Although most of them had a rhythmic activity limited to the flexor bursts, some fired throughout locomotor sequence with a maximal rate during flexor bursts. All the 123 E neurones completely stopped firing during the flexor phase. As was the case for F3 neurone firing, E3 neurone firing (34 neurones) occurred only during periods of locomotor activity. Among the neurones that displayed tonic activity between locomotor episodes, the E2 neurones (24 from 123) remained at this resting value during the extensor phase whereas the E1 neurones (65 neurones) showed an increased i.f. for all or part of this phase. These data, which suggest an asymmetrical genesis of the flexor and extensor activities in locomotion, need to be supported by further analysis.