The effects of tonic pain stimulation on heteronymous Ib pathways from the gastrocnemius medialis (GM) to the soleus (Sol) and to the quadriceps (Q) muscles were investigated in four healthy human subjects. Tonic pain stimulation was performed by subcutaneous injection of 0.5 mg levo-ascorbic acid or vitamin C (L-LAS) in a volume of 0.5 ml on the dorsal surface of the ipsilateral foot. The mean curve of L-AS-induced pain sensation showed a steep rising phase reaching maximum intensity at 2-3 min, followed by a slow decay phase lasting about 15-20 min. Between about 5 and 20 min after injection, there was evidence of pure pain stimulation due to chemical activation of free nerve endings. During this interval, significant potentiation of Ib inhibition from GM to both Sol and Q motoneurones was observed. The time-course of these Ib heteronymous changes paralleled that of subjective pain sensation. These findings demonstrate that nociceptive discharge modifies the gain of Ib heteronymous effects in humans. Since the man function of these Ib pathways is to coordinate activity of muscles operating at different joints, it is suggested that nociceptive input may change muscle synergies by selecting specific subpopulations of Ib interneurones, thus contributing to establish appropriate adaptive motor strategies.