The GABA-transaminase inhibitor, gamma-vinyl-GABA (vigabatrin), has 100% oral bioavailability and is used orally to treat some forms of epilepsy (Goldsmith & de Bittencourt, 1995; Richens, 1992). There are no reports on the mechansisms involved in vigabatrin transport across the small intestinal wall. We have recently identified that the H⁺-coupled amino acid transporter (system PAT), characterised functionally using human intestinal Caco-2 cell monolayers (Thwaites et al. 2000), represents the classical “imino acid carrier” in both human and rat small intestine (Anderson et al. 2004). A system PAT-related cDNA clone was isolated from a Caco-2 cell cDNA library and named hPAT1 for human Proton-coupled Amino acid Transporter 1 (Chen et al. 2003). The purpose of this investigation was to determine the role of system PAT/hPAT1 in vigabatrin absorption. Radiolabelled uptake experiments ([³H]GABA, 20-100μM, 0.5-5.0μCi.ml^-1) were performed using Caco-2 cell monolayers (passage no. 108-118, 14-17 days post-seeding, 15min at 37^oC) and hPAT1-expressing Xenopus laevis oocytes (cRNA 50ng.oocyte^-1, 2 days post-injection, 40min at 23^oC). Under optimal transport conditions (pH 5.5, Na⁺-free buffers) vigabatrin (10mM) caused a significant reduction in [³H]GABA uptake (mean ± sem (n)) across the apical membrane of Caco-2 cells (from 728±76 (6) to 271±14 (6) pmol.cm^-2, p〈0.001, ANOVA Bonferroni post-test). Inhibition of [³H]GABA uptake via unlabelled GABA and vigabatrin had a similar concentration dependence producing IC₅₀ values of 2.1±1.4 and 2.8±1.6 mM, respectively. In hPAT1-expressing oocytes vigabatrin reduced [³H]GABA uptake by 55.8±12.3 (9) %. To determine whether vigabatrin was a transport inhibitor or substrate, substrate-induced current flow into hPAT1-expressing oocytes was measured under voltage-clamped conditions, where both GABA and vigabatrin (both 10mM) induced similar current flow (the response to vigabatrin was 75.7±6.9 (3) % (mean ± sd) of that with GABA). Similar observations were made using the GABA analogues TACA (transaminocrotonic acid) and guvacine. No current was observed in water-injected oocytes. In conclusion vigabatrin is transported across the apical membrane of Caco-2 cell monolayers via the high capacity, H⁺-coupled transporter hPAT1, which emphasises the potential of this transporter as a route for oral drug delivery.