Antispastic effects of the noradrenaline and dopamine precursor l-3,4-dihydroxyphelanine (L-dopa) were investigated in 11 subjects in which exaggerated stretch reflexes developed after spinal cord injuries. The effects were evaluated from changes in the electromyographic (EMG) response of the quadriceps muscle during tendon jerks evoked by standardized taps over the patellar tendon, in clonus and in resistance to passive movements of the limb. After administration of L-dopa, EMG responses occurring 30-150 ms after the tendon tap decreased to about 50% of control, and clinical tests revealed a marked decrease in the resistance to muscle stretches and in the degree of clonus. The effects were maximal within about 1 h. The depressive actions of L-dopa are interpreted as being exerted primarily at the spinal level, since they were evoked in paraplegics and tetraplegics. The results support the previous hypothesis that group II muscle afferents contribute to.the exaggerated stretch reflex in spastic patients because L-dopa depresses transmission from group II but not from group I muscle afferents. They also indicate the possibility of using L-dopa in the treatment of spastic patients.