The excitability of a lower limb myotatic reflex arc is modulated by the antecedent activation of another myotatic arc in the same limb; the changes can be represented by heteronymous excitability curves. This work compares heteronymous excitability curves in spastic and normal subjects. Eighteen patients who showed clear signs of pyramidal tract lesions at a chronic stage were studied. Three myotatic arcs of the lower limb (soleus, quadriceps, and short biceps) were activated either by mechanical percussion (T Sol-T Quad-TSBi) or by electrical stimulation of the afferent nerve (H Sol-H Quad). They were used as conditioning and test responses in six different combinations, and the results compared to those obtained previously in normal volunteers. In spastic patients, heteronymous excitability curves are more obviously modified than homonymous ones. The changes are most prominent in the thigh muscles and less marked when the soleus reflex is conditioned by activation of quadriceps and biceps femoris myotatic reflex arcs.