A further attempt has been made to define the receptor responsible for the inhibition of monosynaptic reflexes by vibration in man. Vibration of the tendo Achillis will produce inhibition of the H reflex even when the muscles of the anterior compartment of the leg are denervated or blocked with local anaesthetic, implying that there are receptors in the posterior compartment capable of producing this effect. However, there is evidence that vibration spreads through the limb. The inhibition is greater when the anterior compartment is innervated indicating that there is a contribution from receptors in this compartment. Stretching the muscles of the posterior compartment alone, or the muscles of the anterior and posterior compartments reciprocally does not influence the inhibition of the monosynaptic reflex by vibration. These observations support the contention that the reduction of the monosynaptic reflex by vibration in man is due to presynaptic inhibition resulting from activation of primary spindle endings.