Variations in the H-reflex of soleus, tibialis anterior, quadriceps and short head of biceps femoris muscles were studied in normal man, preceded by a conditioning stimulus applied to either gastrocnemius medialis, the inferior branch of soleus or the common peroneal nerve. The observed variations were evoked by low threshold afferents, had a very short latency suggesting oligosynaptic linkage and disappeared during a selective ischaemic blockade of group I fibres; they were therefore regarded as due to group I fibre stimulation. Biphasic variations were observed in almost all cases: evidence is presented that the early low threshold effects, facilitatory for synergistic and inhibitory for antagonistic motoneurone pools, are due to stimulation of Ia fibres, while the subsequent opposite effects which had a slightly higher threshold are Ib in origin. Even though the pattern of projections of group I fibres from ankle flexors and extensors is qualitatively the same in normal man as in the spinal cat, large quantitative differences are observed; in normal man the effects of group I fibres from ankle flexors and extensors upon close synergistic and direct antagonistic motoneurone pools are weak, and often negligible, whereas they are extremely potent upon quadriceps motoneurones.