Reciprocal Ia inhibition from ankle flexors to extensors was studied during voluntary tonic isometric dorsiflexion and plantar flexion in five normal subjects. The Ia inhibition was examined as the short-latency suppression of the soleus H-reflexes by stimulation of the low-threshold afferents in the common peroneal nerve (Mizuno et al. 1971). At rest, weak Ia inhibition was demonstrated in four subjects out of five, the maximal amount being 14.1 +/- 5.0% suppression of the control H-reflex. The absolute amount of inhibition, which was calculated by subtracting the mean size of the conditioned H-reflex from that of the control H-reflex and expressed as a percentage of the maximal M-response, increased during ankle dorsiflexion, and decreased or disappeared during plantar flexion in parallel with the amount of contraction. The neural mechanisms for facilitation of the Ia inhibitory pathway during dorsiflexion were considered to support the hypothesis of "alpha-gamma-linkage in reciprocal inhibition", i.e. combined facilitatory effects on the Ia inhibitory interneuron from the supraspinal centers directly and indirectly via the gamma motoneuron--Ia afferent route. The mechanism for inhibition of the pathway during plantar flexion was considered to be inhibition of the Ia interneuron of the flexor side by Ia interneuron of antagonist extensors. A quantitative aspect of activity in the reciprocal Ia inhibitory pathway on the performance of voluntary movement is revealed in this study.