Six patients with long-lasting spasticity resistant to different drug therapies including oral baclofen received a bolus injection of lumbar intrathecal baclofen. Electromyographic (EMG) reactions of leg muscles (soleus, tibialis anterior, quadriceps, and hamstrings) to standard stimuli and during attempts at voluntary activation were recorded before the drug injection and up to 3 h after the injection. Responses to joint movements, H-reflexes, ankle clonus, and defensive reactions were noticeably suppressed within 30-45 min after the injection and had practically disappeared after 2 h. Ankle clonus was seen only in patients with H-reflexes, and clonus disappeared when the reflex responses to the n. tibialis stimuli were absent. A decrease in clonus EMG burst amplitudes was accompanied by a decrease in the clonus frequency. These observations favor the autooscillation hypothesis of clonus. Baclofen injection led to improvement in selective voluntary activation of leg muscles in patients with residual motor control. These results suggest that execution of voluntary motor commands in the patients suffered from functionally abnormal spinal circuitry rather than from changes in the descending motor commands. Intrathecal baclofen appears to be an effective way of eliminating increased muscle tone and spasms which can allow for voluntary motor function when it is present.