Effects of low threshold afferents from the brachioradialis (BR) to the flexor carpi radialis (FCR) motoneurone pool in humans were examined using an electromyogram-averaging (EMG-A) technique. The experiments were performed in 10 healthy human subjects. Electrical (ES) and mechanical conditioning stimuli (MS) were delivered to BR during recording of EMGs of FCR exhibiting steady tonic contraction (isometric contraction) with the level of 10% of the maximal contraction. EMGs were rectified and averaged. Fluctuation of EMG-A after the stimuli was compared with those of EMG-A 10-60 ms before the stimuli (control, 100%). ES (rectangular pulse, width 1.0 ms) was delivered to the BR nerve with an intensity immediately below the motor threshold and MS (quick tap, 0.4-1.8 mm depth within 5 ms) was to the distal tendon of BR with that below the threshold of the tendon(T)-wave. Early and significant troughs in EMG-A were produced by ES and MS in all the 10 subjects. The amplitudes of the troughs by ES and MS were 84-88% and 83-89%, respectively, of those of the control EMGs. The latency and duration of the troughs by ES were 12.0-16.0 and 3.4-6.0 ms and those by MS were 17.2-20.6 and 4.0-6.4 ms, respectively. The difference between latencies of the troughs by ES and MS was almost equivalent to that of the Hoffmann- and T-waves of BR. In 3 subjects, the trough was diminished by tonic vibration stimuli to BR. These findings suggest that group Ia afferents of BR inhibit an excitability of the FCR motoneurone pool.