The facilitation of the quadriceps (Q) H reflex evoked by stimulation of group I afferents from pretibial flexors exhibits biphasic changes during Q voluntary contraction. At short conditioning-test intervals the facilitation is increased, whereas it is decreased at longer intervals and/or at high conditioning stimulus intensities and/or when the contraction strength is increased. The spatial facilitation at a premotoneuronal level observed at rest on combined stimulation - common peroneal nerve (CPN) and femoral nerve (FN) - regularly disappeared during contraction. It is argued that the increase in facilitation at the onset of the weakest Q contraction reflects a descending facilitation of the interneurones mediating the CPN-induced excitation to Q MNs. An occlusion of descending and peripheral excitation in these interneurones can easily account for the depression of the facilitation when the conditioning stimulus intensity and/or the strength of the contraction is increased. However, occlusion cannot explain all the results and inhibition of the neurones mediating excitation of MNs, i.e. disfacilitation of the MNs, probably contributes to the decrease in facilitation observed during contraction.