A method to assess changes in presynaptic inhibition of I a afferent terminals in man is proposed. The soleus H reflex was facilitated by a heteronymous I a volley from quadriceps and the amount of reflex facilitation was used to estimate the size of the conditioning I a excitatory post-synaptic potential (e.p.s.p.). It is argued that the size of this e.p.s.p. as measured by the resulting amount of reflex facilitation reflects the amount of presynaptic inhibition on the corresponding I a fibres. A decrease in the reflex facilitation may then be ascribed to an increase in presynaptic inhibition of the I a fibres mediating the conditioning volley. 2. That the heteronymous I a facilitation from quadriceps to soleus is caused by a purely monosynaptic e.p.s.p. is a prerequisite for the validity of the method. Experimental evidence is therefore given in an Appendix that in man the earliest part (first 0.5 ms) of this heteronymous I a facilitation is mediated through a monosynaptic pathway. Evidence is also given that this earliest facilitation is not yet contaminated by any polysynaptic effects from I a or I b afferents. 3. The validity of the method was established in animal experiments in which presynaptic inhibition of I a fibres and post-synaptic events in motoneurones could be assessed by direct tests. It was found that the amount of test reflex facilitation produced by a conditioning I a volley was decreased when I a fibres were subjected to presynaptic inhibition but remained unchanged when the motoneurone pool in which the test reflex was elicited received pure post-synaptic inhibition. 4. In man, presynaptic inhibition of I a fibres was evoked by a short-lasting (three shocks at 200 Hz) vibration applied to the tibialis anterior tendon. Such a vibratory burst reduced the efficiency of the heteronymous I a volley in facilitating the soleus H reflex. By contrast, during a pure post-synaptic inhibition of soleus motoneurones the efficiency of the conditioning volley in facilitating the test reflex remained unchanged. It is therefore argued that the amount of heteronymous I a facilitation can indeed be used to assess the amount of ongoing presynaptic inhibition exerted onto heteronymous I a fibres from the quadriceps muscle to soleus motoneurones. 5. The short-lasting tibialis anterior vibration used here evoked a long-lasting (300-500 ms) depression of soleus and quadriceps H reflexes.(ABSTRACT TRUNCATED AT 400 WORDS)
