Abstract Stimulating electrodes were stereotaxically implanted in the substantia nigra of adult cats. After recovery from surgery, stimulation with constant current square-wave pulses produced motor behavior similar to that obtained upon activation of striatal structures. The initial response was an “arrest” reaction, which subsequently involved head turning and finally contraversive circling movements as the frequency of stimulation was increased. All of these stimulus-induced responses were antagonized by either haloperidol or chlorpromazine. It is suggested that these behavioral responses are ultimately mediated by the reticulospinal system.