Tonically firing gamma motoneurones of known conduction velocity (total eighty-seven, range 15-43 m/sec) have been isolated in peripheral muscular nerves to triceps surae. Their responses to stretch of triceps surae have been studied in decerebrated cats. A small amplitude, quick stretch and release was used to provide a selective stimulus for primary endings of muscle spindles. 2. To check the selectivity, recordings were made from 135 afferents from triceps surae under conditions closely similar to the reflex experiments. The threshold of all but a few primary endings of muscle spindles law below 50 micrometer whereas threshold was above 50 micrometer for the majority of secondary endings and tendon organs. A 20 micrometer stretch excited approximately half the primary endings but only one of thirty-six secondaries and no tendon organs responded to such a small stretch. Nine group III afferents were also studied but none responded to stretch. 3. Stretch of up to 50 micrometer excited twenty-three and inhibited eleven gamma motoneurones while thirty-three remained unaffected. A further twenty showed mixed responses, being inhibited initially before being excited at longer latency. Thresholds for reflex responses of gamma motoneurones frequently occurred below 20 mum and responses were close to maximal for stretch of 50 micrometer. 4. Excitation always had a lower threshold to stretch than did inhibition for those gamma motoneurones showing mixed responses and was the more potent of the two effects. 5. Excitation to stretch had central delays, to the incoming group Ia volley, ranging from 5 to 14 msec while similarly calculated delays for excitation of alpha motoneurones ranged from 0.6 to 3.0 msec. Central delays of the gamma inhibitory responses lay in an intermediate range of 1.7-7.0 msec. 6. The long central delays of excitation of gamma motoneurones in response to stretch do not reflect transmission in supraspinal pathways since the reflex persisted following spinal section. 7. Excitation of gamma motoneurones was weak in comparison with that of tonically firing alpha motoneurones recorded in the same preparations and it was always necessary to sum a number of responses in order to reveal an effect...
