Gabapentin (GBP) has been shown to reduce paired-pulse inhibition in the dentate gyrus of the urethane-anesthetized rat, which is a proconvulsant effect, and to shorten the afterdischarge duration, which is an antiepileptic effect. The mechanism by which GBP exerts these effects is not known, but a number of possibilities have been proposed. Here we tested the ability of vigabatrin (VGB), a GABA transaminase inhibitor, and SKF89976A, a selective GAT-1 blocker, to alter the effectiveness of GBP in the dentate gyrus in urethane-anesthetized adult Sprague-Dawley rats. VGB, alone at 100 mg/kg, had no effect on the evoked potentials or paired-pulse inhibition in the dentate gyrus, but did block lengthening of the afterdischarge. Pretreatment with VGB had no effect on the ability of GBP to reduce paired-pulse inhibition, but blocked the effect of GBP on seizure duration. SKF89976A, alone at 10 mg/kg, increased paired-pulse inhibition and blocked the lengthening of the afterdischarge in the seizure model. Pretreatment with SKF89976A had no effect on the actions of GBP on either paired-pulse inhibition or seizure duration. These results suggest that the action of GBP is not mediated through an inhibition of the GAT-1 transporter and probably not through an increase in basal levels of GABA. The data also suggest that the combination of VGB and GBP may be clinically less effective than the use of GBP alone.