We studied the effect of botulinum toxin A injection on the abnormal presynaptic phase of reciprocal inhibition between forearm antagonist muscles in patients with essential tremor. Ten patients with essential tremor were investigated before and 1 month after botulinum injection. Reciprocal inhibition was studied by conditioning the H reflex in forearm flexors with a radial-nerve stimulus delivered at a range of time intervals. Botulinum toxin produced a significant functional improvement in tremor (about 20%). Before botulinum toxin injection, patients had a reduced presynaptic phase of reciprocal inhibition. After botulinum toxin this phase was significantly more pronounced. The normal early disynaptic phase of reciprocal inhibition was normal before and after botulinum treatment. Although botulinum treatment reduced the size of the H reflex and the M wave to a similar extent, it left the H/M ratio unchanged. These findings show that botulinum toxin treatment restores presynaptic inhibition between forearm antagonist muscles. The results are also consistent with botulinum toxin having a beneficial effect in patients with essential tremor. Both effects probably depend upon the toxin's concurrent action on the extrafusal and intrafusal motor end-plates, the latter resulting in decreased spindle afferent input to the spinal cord.