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Purpose: Vigabatrin (VGB) is an irreversible inhibitor of γ-aminobutyric acid (GABA) transaminase. Its use as an antiepileptic drug (AED) has been limited because it causes retinal dysfunction, leading to visual field defects (VFDs). We performed this study to identify factors contributing to acute VGB retinotoxicity. Methods: In ex vivo experiments, Sprague–Dawley rat retinas were isolated and incubated with VGB or GABA in the presence or absence of light. In in vivo experiments, Sprague–Dawley rats were given intraperitoneal injections of VGB and then exposed to light or kept in the dark. The retinas were analyzed histologically by using both light and electron microscopy. Results: Incubating retinas with 50–500 μ M VGB under 20,000 Lux white light for ≤20 h caused a characteristic time- and dose-dependent degeneration limited to the outer retina. Incubating retinas with 500 μ M VGB in darkness for 20 h caused no damage. Five hundred micromolar GABA and 50 μ M tiagabine were not toxic in the presence or absence of light. Sprague–Dawley rats exposed to an intense white light for 20 h after a 1,000-mg/kg intraperitoneal injection of VGB showed damage in the outer retina, whereas those kept in the dark did not. Conclusions: Direct exposure of the retina to VGB causes acute retinotoxicity that depends on light exposure rather than GABA accumulation. [ABSTRACT FROM AUTHOR]