The peripheral mechanisms concerned in the reduction of monosynaptic spinal reflexes in the cat following injection into the blood stream of decamethonium and succinylcholine were investigated. Both extensor and flexor monosynaptic reflexes are reduced by these drugs. Through a series of experiments designed to detect the effect of these drugs on peripheral sensory endings and the effect of exclusion or isolated inclusion of the muscle whose reflex was being studied, it was determined that the reduction was dependent upon intact neuromuscular innervation, there being little central effect. Changes in activity from muscle spindles are the likely ultimate peripheral cause of this suppressant action.