Following the demonstration that lysergic acid diethylamide (LSD) inhibits myoclonic responses to intermittent photic stimulation (ILS) in the baboon, some other hallucinogenic drugs and some " non-hallucinogenic" derivatives of tysergic acid have been tested for their effects on the EEG and on photically induced epilepsy in twenty-two adolescent baboons (Papio papio), Psilocybin (I-4 mg/kg) and dimethyltryptamine (2-4 mg/kg) produced marked autonomic effects, EEG changes consistent with a severe disturbance of consciousness and a complete abolition of myoclonic or paroxysmal EEG responses to ILS (thus closely resembling the effect of LSD, 40-100/~g/kg). Mescaline (up to 32 mg/kg) was relatively ineffective on both the background EEG and on the myoclonic responses to ILS. After methysergide (2-5 mg/kg), although the autonomic effects were less severe than after psilocybin, there were muscular hypotonia, an abnormal EEG and reduction or abolition of the epileptic responses to ILS. BOL 148 and methergoline appeared less potent than methysergide, but, at 4 mg/kg, both drugs abolished ILS-induced myoclonus. Averaged responses to flash stimulation showed that psilocybin resembled LSD 25 in modifying transmission in the afferent visual pathway, whereas methysergide, in doses blocking ILSinduced myoclonus, did not alter primary evoked responses in the visual cortex. It is suggested that a serotoninergic system is probably involved in the epileptic responses of the baboon to ILS.