The role of proprioceptive input from the ankle extensor triceps surae in the control of walking was examined in premammillary cats walking on a treadmill. The left hindlimb was rigidly fixed in one position after denervating almost all the leg muscles except the ankle extensor (triceps surae) and ankle flexor (tibialis anterior). Rhythmic alternating contractions of the isolated ankle flexor and extensor occurred in the fixed hindleg during periods of walking in the other three intact limbs. These rhythmic contractions disappeared when the isolated triceps surae was stretched so as to increase the force of the active contractions to beyond 4 kg. With maintained stretch the periodic contractions in the ankle flexor and extensor returned only after the force in the stretched triceps surae gradually decreased and fell below approximately 4 kg. Isometric contractions of the triceps surae produced either by stimulation of ventral root S1 or by large amplitude vibrations also led to the sudden disappearance of ankle flexor bursts. Inhibition of the locomotory rhythm could also be produced in all muscles of a single intact hindleg by clamping the ankle joint in a flexed position so as to stretch the ankle extensor. In all these cases, an increased rate of stepping of the contralateral hindleg was associated with the inhibition of the rhythmic locomotory activity. It is suggested that triceps surae proprioceptors signalling the presence of loading of the hindlimb extensor muscles inhibit the central generation of hindlimb flexion. During normal walking this mechanism could be of major importance during stance to prevent the initiation of the swing phase of a time when hindlimb extension is fully needed to support the weight of the animal. Thus a necessary, but not always sufficient, condition for the initiation of swing may be an unloading of leg extensor muscles.