We studied the ratio of the maximal H-reflex to maximal direct muscle potential (H/M ratio), late facilitation and late inhibition in the recovery curve, and vibratory inhibition of the soleus H-reflex in three consecutive patients with hereditary dopa-responsive dystonia, before and during treatment with levodopa. In one patient, we repeated the H-reflex tests twice after withdrawal of levodopa. The results were compared with those in a group of 48 healthy subjects. In the patients before treatment, the soleus H-reflex recovery curve showed increased late facilitation and depressed late inhibition, reflecting alterations in postsynaptic interneuronal activity. Vibratory inhibition, predominantly reflecting presynaptic inhibitory action, was depressed. Normalization of these test results occurred during levodopa treatment, concurrent with a clear clinical response. The H/M ratio, reflecting the excitability state of the mo-toneuron pool, was similar during and without levodopa treatment. In the one patient tested after levodopa withdrawal, enhancement of late facilitation and decrease of vibratory inhibition paralleled the reoccurrence of dystonia most clearly. Since soleus H-reflex tests mainly reflect mechanisms operating at the spinal level, spinal aminergic or dopaminergic systems are probably involved in dopa-responsive dystonia.