Vigabatrin (gamma vinyl GABA, GVG) has been shown to be an effective antiepileptic agent. GVG specifically and irreversibly inhibits GABA-transaminase (GABA-T). Long-term animal toxicology studies have demonstrated that GVG can induce nonprogressive, reversible intramyelinic edema in central white matter tracts. The response to GVG varies among species, with rodents being the most dramatic and monkeys showing an equivocal effect even at high doses. The response in dogs is marked and measurable. The detection of these subtle findings requires the use of sophisticated technology. Evoked potentials are becoming reliable and sensitive tools in clinical neurology. This study, involving 54 patients for 11 months, was undertaken to assess the effect and safety of GVG in humans with refractory epilepsy. No data from this investigation indicate prolongation of neuronal conduction time in CNS pathways, suggesting that this agent is safe in humans.