We conducted two types of experiments to assess the validity of the H-reflex recovery test, using double stimulation to test soleus motoneuron pool excitability in healthy and spastic subjects. One type dealt with the mechanical effect of the conditioning H reflex on the ankle joint; the other type with the effect of change in reflex size. The mechanical effect was tested both with the ankle joint fixed (FX) and free to move (FR). Differences between FX and FR conditions commenced with relaxation of soleus muscle contraction by the conditioning H reflex. In the FR condition, abrupt facilitation occurred, and changed to marked depression. We conclude that specific facilitation and inhibition in the FR condition were secondary effects of group Ia inflows caused by the ankle extensor muscle stretching on relaxation. In some spastic patients as well as in controls, facilitation due to the mechanical effect in the FR condition was observed despite the FX condition. The effects of systematic changes on soleus H-reflex size were investigated at conditioning-test intervals of 80 ms, so as to avoid mechanical effects. When conditioning and test reflexes were the same size, the amount of recovery increased as the H-reflex size increased. Comparison of the relation between amount of recovery and H-reflex size, expressed as a percentage of Mmax, showed no significant difference between the two groups. We speculate that the stronger recovery of spasticity mentioned in previous literature may have resulted from the fact that relatively greater H reflexes were tested in those studies. In conclusion, the present study indicates that double stimulation is not appropriate for assessing spinal motoneuron pool "excitability increase" in spasticity.