1. The aim of this study has been to determine the autogenetic reflex effects of tendon organ stimulation upon gamma motoneurones in the decerebrated cat. 2. Tendon organs serving a muscle were excited by isometric twitch contractions of some motor units of that muscle. Contractions were elicited by electrical stimulation of the cut peripheral end of part of the ventral root supply, using a stimulus strength at or below maximum for alpha motoneurone axons. Afferent recordings in the same cats as were used to study reflexes of gamma motoneurones showed that tendon organs were powerfully excited during twitch contractions of their muscle. Early discharges were also elicited in both tendon organs and spindle primary afferents. 3. Background discharges of forty-seven triceps surae gamma motoneurones, ranging in axonal conduction velocity from 17 to 40 m/sec, were recorded from filaments of the muscle nerve. The initial response of twenty-two (47%) neurones to twitches of the triceps surae was inhibition of their discharge. The latency from time of stimulation of the ventral root ranged from 10 to 22 msec. The lowered probability of firing lasted an average of 25 msec and, occasionally, was seen as a short silent period. 4. The inhibition was reduced in duration by decreasing the resting muscle length so that less active tension was developed during contraction. 5. The inhibition was observed, in one instance, in the spinally transected decerebrated cat. 6. We argue the case that early discharges in tendon organ afferents give rise to the onset of inhibition and that tension dependent discharges of tendon organs contribute further inhibition of gamma motoneurones. 7. Sixteen gamma motoneurones were not affected by contraction but seventeen showed a period of facilitation. The facilitation could occur after a period of inhibition (eight neurones) or as the sole response to contraction. In either case facilitation occurred mainly during relaxation of the muscle and may, in part, be attributable to increased spindle primary ending firing at this time. 8. In discussion a comparision is made of the reflex effects of muscle afferent discharges on alpha and gamma motoneurones.