Intravenous injection of 5-hydroxytryptophan (5-HTP) excites discharges of gamma motoneurones to extensor muscles of the hind limbs of decerebrate spinal cats and this occurs with a short latency (1-2 min). 2. 5-HTP also facilitates the stretch reflex in the spinal cat but, in contrast, there is a latency of about 10 min with a maximum action after 20 min. 3. Facilitation of the stretch reflex by 5-HTP still occurs when the gamma loop has been opened by cutting all ventral roots supplying the muscle. Thus excitation of gamma motoneurones by 5-HTP is not required for facilitation of the stretch reflex. 4. A possible contribution to the stretch reflex from the increased discharge of gamma motoneurones has been assessed. Selectively removing gamma activity rarely altered the reflex response to stretch and in only 3 out of 15 spinal cats could we conclude that gamma discharges contributed towards facilitation of the stretch reflex by 5-HTP; 5; Methysergide (0.25 mg/kg) blocked completely the facilitation of the stretch reflex caused by 5-HTP in the spinal cat but only partially antagonised the reflex in the decerebrate cat, even when injected intra-arterially close to the lumbar spinal cord. 6. Lysergic acid diethylamide (LSD) was found to mimic the actions of 5-HTP. It facilitated the stretch reflex and excited extensor gamma motoneurones in the spinal animal.
