1. Heteronymous group II effects were investigated in the human lower limb. Changes in firing probability of single motor units in quadriceps (Q), biceps (Bi), semitendinosus (ST), gastrocnemius medialis (GM) and tibialis anterior (TA) were studied after electrical stimuli between 1 and 3 times motor threshold (MT) applied to common peroneal (CP), superficial (SP) and deep (DP) peroneal, Bi and GM nerves in those nerve-muscle combinations without recurrent inhibition. 2. Stimulation of the CP and Bi nerves evoked in almost all of the explored Q motor units a biphasic excitation with a low-threshold early peak, attributable to non-monosynaptic group I excitation, and a higher threshold late peak. When the CP nerve was cooled (or the stimulation applied to a distal branch, DP), the increase in latency was greater for the late than for the early peak, indicating that the late excitation is due to stimulation of afferents with a slower conduction velocity than group I fibres, presumably in the group II range. In ST motor units the group II excitation elicited by stimulation of the GM and SP nerves was particularly large and frequent, and the non-monosynaptic group I excitation was often replaced by an inhibition. 3. A late group II-induced excitation from CP to Q motoneurones and from GM and SP to ST motoneurones was also observed when using the H reflex as a test. 4. The electrical threshold and conduction velocity of the largest diameter fibres evoking the group II excitation were estimated to be 2.1 and 0.65 times those of the fastest Ia afferents, respectively. In the combinations tested in the present investigation the group II input seemed to be primarily of muscle origin. 5. The potent heteronymous group II excitation of motoneurones of both flexors and extensors of the knee contrasted with the absence of a group II effect from DP to GM and from GM to TA. In none of the combinations explored was there any evidence for group II inhibition of motoneurones. The possible contribution to postural reactions of the potent group II excitation of thigh motoneurones is discussed.