The effects of intracerebroventricular (i.c.v.) or systemic injections of Met- or Leu-enkephalin, β-endorphin, FK 33.824 (D-Ala 2, MePhe 4, Met(O 5)-ol-enkephalin) and of morphine and naloxone have been studied in baboons, Papio papio, which spontaneously show photically induced epileptic responses. Animals were chronically implanted with epidural or deep recording electrodes and a cannula in one lateral ventricle, and tested whilst seated in a primate chair. In some animals the natural syndrome was enhanced by the prior administration of dl-allylglycine, 100–200 mg/kg, i.v. Met- or Leu-enkephalin, 1–10 mg, i.c.v., did not lead to any manifest focal or generalized seizure discharges. Nor did it lead to any consistent enhancement or reduction of photically induced myoclonic responses (as tested 5–10 min after injection). β-Endorphin, 0.1–0.5 mg, i.c.v., did not enhance or impair photically induced myoclonic responses. FK 33.824, 0.1–0.5 mg, i.c.v., depressed respiration and slowed EEG background rhythms for 9–15 h. This was associated with a loss of myoclonic responses to photic stimulation. These effects were reversed for 20–40 min following the injection of naloxone, 1 mg/kg i.m. A depression of respiration and a slowing of EEG rhythms was seen beginning 5–20 min after FK 33.824, 2 or 4 mg/kg, i.v. The higher dose also abolished photically induced myoclonic responses. Naloxone, 1 mg/kg, definitively reversed these effects. Morphine, 5–10 mg i.c.v., tended to increase the latency to onset of generalized myoclonus during photic stimulation. Myoclonic responses were delayed or diminished after morphine, 5 mg/kg, i.m. Naloxone, 1–2 mg/kg i.m., reversed this effect. Naloxone, 0.2–5.0 mg/kg i.m., alone, did not significantly modify photically induced myoclonus, either in animals of low or high initial responsiveness, or in those pretreated with allylglycine.