Eighteen of the first 29 patients with intractable epilepsy treated by chronic cerebellar stimulation (CCS) demonstrated a marked suppression of seizures. Sixty-eight of 100 patients with cerebral palsy showed clinical improvement after CCS. Electroencephalographic studies in three epileptic patients revealed a significant (P less than 0.001) reduction in number and duration of paroxysmal EEG discharges during epochs when the stimulator was on; prolonged effects were seen at stimulation rates of 200 c/sec and 10 c/sec (monophasic capacitively coupled stimuli). "Rebound" increases in numbers and durations of paroxysmal discharges occurred after cessation of CCS: immediate "rebounds" occurred within the next 5 min; such rebound effects were also seen in the frequency of clinical seizures. CCS at voltages well above threshold for the production of changes in H reflexes, late motor responses (V1 and V2), and evoked potentials resulted in increased "rebound" effects after cessation of stimulation and such effects were seen clinically and neurophysiologically in epileptic and cerebral palsy patients. Variability in the effects of CCS on seizures and the EEG may have been due to technical factors such as positions and impedances of electrodes, output of the stimulator, effects of anticonvulsant medication and patient differences; there was no clinical or physiological evidence of any undesirable neurological effect of CCS. In one patient, onset of CCS was frequently associated with cessation of polyspike and wave discharges; such results raise the possibility of triggering CCS from paroxysmal discharges in the EEG (contingency feedback) but rebound effects may complicate such therapy.