The effects of stimulating the cerebellar surface on abnormal segmental reflexes were examined in monkeys rendered spastic by either bilateral or unilateral decortication of areas 1, 2, 3, 4 and 6. Rectified, integrated electromyographic (EMG) activity from the biceps and triceps as well as the torque were recorded and averaged during successive flexion-extension movements of the arm produced by a displacement controlled torque motor. Two movement paradigms were employed. The first consisted of a ramp and hold paradigm with an initial rapid flexion of the forearm, a subsequent two second period during which the arm position was held constant, and a rapid extension returning the limb to the initial position. In the second paradigm, the forearm was modulated sinusoidally at several different frequencies. Cerebellar stimulating electrodes were placed over the paravermal region bilaterally. Different frequencies (10 to 300 HZ) and different charge densities (1.5 to 10 microC/cm2) were used in each animal. In each experimental trial short periods of cerebellar stimulation (10 to 30 minutes) were interspersed between control periods. As observed in some types of clinical spasticity, flexion or extension of the extremity was capable of evoking coactivation of the biceps and triceps. Cerebellar surface stimulation reduced the amplitude of the phasic and tonic stretch reflexes recorded from the triceps during flexion and decreased the abnormal triceps response during passive shortening. The biceps response to stretch was increased by cerebellar stimulation and its abnormal response to flexion was decreased. These combined effects modified the organization of the segmental reflexes, producing a more normal reciprocal relationships of the EMG activity in the biceps and triceps. Evaluation of many different combinations of stimulus parameters revealed that not only the magnitude of the passive reflex but also the nature of the effect was dependent on stimulus parameters. Although these studies demonstrate an improvement in the abnormal reflexes present in spastic primates, they emphasize the complexities of the response evoked by cerebellar stimulation and the importance of stimulus parameters in the modifications produced by this technique.