Summary:Vigabatrin (gamma vinyl GABA, GVG), an enzyme‐activated, irreversible inhibitor of GABA transaminase, was administered orally to rats, dogs, and monkeys to observe toxicologic reactions. Myelin vacuolation of the brain was observed. The vacuolation was limited to myelinated tracts and resulted from separation of the myelin sheath at the interperiod line. There was no evidence of demyelination, axonal degeneration, or damage uolation was histologically similar to that observed in association with other drugs such as triethyltin, isoniazid, or hexachlorophene. However, the distribution is limited to the brain and is reversible upon discontinuation of therapy. Two postmortem and three operative specimens from humans have revealed no evidence of vacuolation of myeli